Hi and welcome to GMAT Club!
Yes, there are differences between tests. Definitely. Many feel that
MGMAT has harder math than what you will encounter on the GMAT.
Others tend to feel Kaplan was harder as well. There is not as much about Veritas Prep but most feel it is close. similarly
GMAT Club tests for example is harder than official questions you encounter
The bottom line is the scoring system - e.g. does
MGMAT allow you to make more errors potentially and still get a certain score. Similarly an easy question provider (e.g. Princeton Review) can still give you a low score if you miss just a handful of questions vs.
MGMAT giving you a higher score even with missing half. So, it is not only about the difficulty of questions but also about the scoring of the whole test.
Hope this helps. You can also see this tool: gmat score calculator (your question is the reason we created it)