There are a few relevant pieces of info to help answer your question.
1. The May 2020 Flex test is the only Flex test for which an official scoring scale has been released.
2. Scoring scales for released tests before May 2020 could differ a bit from test to test. This is what the "curve" is referring to - a test with a "-10" curve is one where 10 incorrect answers would suffice for a score of 170, for example. So getting 10 questions wrong on a practice test could sometimes be good enough for a 170, sometimes for a score slightly higher or slightly lower than that. There's some good info about this variation (on tests before the Flex was introduced!) here: https://www.powerscore.com/lsat/help/scale.cfm
3. Practice tests released before the May 2020 test have a scoring scale based on the 4-section nature of those tests. The Flex test has only 3 scored sections. When we convert a practice test into a Flex version, when the original was not Flex, we make some reasonable assumptions about how to convert the scoring to a Flex-like score.
What you're likely seeing is the effect of a different scale on an older test. Although there aren't any other released Flex scales except May 2020 (at this point!), it's very likely the same slight variation exists between different administrations of Flex tests, so this is a variation in your practice that matches the variation in actual test administrations! We have no reason to expect that the May 2020 scale will be the exact scale for ALL Flex tests, for the record.