I don't know what IESE does specifically, or if they do it for every applicant. However, in my experience, when a background check reveals an inconsistency, the school will usually contact the applicant for an explanation. In your case, it sounds as thought the explanation is quite simple, and you can always get a manager from the company to back up your story.
Don't worry... The school won't simply say, "This story doesn't add up... Rejected!" They do these checks to catch BIG lies, such as someone lying about running a whole department or earning a certain GPA in college. A small discrepancy like this, which can be easily explained, won't cause any problems.
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