The idea that you only have one credit score isn't true.

In fact, it's one of the biggest credit score myths people consistently fall for, says John Ulzheimer, a credit expert who formerly worked for FICO and Experian.

"You literally have dozens and dozens and dozens of credit scores that are not all the same three-digit number," he tells CNBC Make It. "You shouldn't assume that just because a website shows you a 750 score that anywhere you go to apply for something is also going to see that 750."

That's because your bank, another lender or a reputable finance website like Credit Karma or Credit Sesame may be using reports from any of the three national credit bureaus — Experian, Equifax or TransUnion — to generate the credit score you're shown, he says.

"It's just a variation of your FICO score," he says. "Maybe one score is being generated from your Experian credit report and another one from your Equifax and since your credit reports aren't the same, your scores aren't going to be the same."

Unfortunately, the difference in numbers can cause confusion or even mistrust if you're not aware that you have multiple credit scores, Ulzheimer says.

"Someone may think, 'I just went to this website that says I have a 750 credit score, but the car dealership is telling me something different so they must be lying to me or trying to charge me more interest,'" he says. "But really, they're not doing anything wrong, they're just pulling a completely different credit report."

Why you should check multiple credit scores

Your FICO score, which is most commonly used by lenders, can fall anywhere between 300 and 850. Here are the FICO score ranges Experian defines as poor, fair, good, very good and exceptional.

• Poor: 300 to 579
• Fair: 580 to 669
• Good: 670 to 739
• Very good: 740 to 799
• Exceptional: 800 to 850

Although you have multiple credit scores, they shouldn't differ from each other too drastically. You shouldn't see one website say your credit score is 810, but another say it's 575 at the same point in time, for instance, Ulzheimer says.

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"What you should see is that all of your scores should be within around 25 to 50 points give or take," he says. "You should not see this wild variation in scores that that clearly tell two very different stories about your credit."

Check your credit report and credit score for free

If you are seeing vastly different FICO scores that fall under different credit tiers, it may be time to check your credit reports for any potential errors, Ulzheimer says.

You have fewer credit reports since there are only three major credit reporting agencies. By law, you're entitled to one free credit report from each of the credit bureaus each year. They also allow you to check your credit report online for free once a week. To do so, you can visit annualcreditreport.com.

And don't worry about those credit report requests negatively impacting your credit score. Checking your own credit report isn't the same as when a lender pulls your credit, so it doesn't effect your score, according to the Consumer Financial Protection Bureau.

It's important to know that your credit report isn't the same as your credit score. The law doesn't entitle you to a free credit score, but there are plenty of ways to monitor it without spending money, such as through your bank or a personal finance site like Credit Karma, Ulzheimer says.

"Most of your lenders are going to give you your credit score for free," he says. "If you're going to go and buy your credit score somewhere, then you're really wasting your money because there's just so many places now where you can get them at no cost if you're willing to ignore a few marketing emails."

While you don't necessarily need to review your credit report or credit score on a weekly basis, having a clear picture of your credit standing can help you better understand what lenders see when you apply for something like a new credit card or a mortgage.

"If you understand what's on your credit report and you understand where your scores are kind of falling generally, you're a more empowered consumer when you go to apply for a loan," Ulzheimer says.

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