Amazon’s official software version page says the latest version is 7.6.4.6 for Gen 2 and Gen 3 Cubes. (So how can you be getting a higher number?) I don’t put much faith in that web page, which is:

https://www.amazon.com/gp/help/customer/display.html?ref_=hp_left_v4_sib&nodeId=G201497590

I have a Gen 2 cube and in my experience the software update functions are a confusing hodgepodge. I never get notifications that an update is available or pending. Sometimes my streaming apps will start acting weird and in settings I will find an update is pending or partly done or something. I just had one of these updates complete and the version numbers before and after the update were identical!

Also, on the Amazon official update page the current update is listed as: FireOS 7.6.4.6 (28085968516) While in my Cube it is listed as FireOS 7.6.4.6 (PS7646/3550) and it has remained the same thru several updates!

Those numbers in parentheses never match between my device and the Amazon listing. I posted a question about this on the official Amazon Fire Cube Forum and received this answer from an Amazon agent:

Sorry to hear that your Fire TV Cube version number isn't matching the current/latest version number. The "7.6.4.6" number is really the only number that you need to reference, Even then, the latest OS version isn't deployed to everyone at the same time, so it's always possible to not have the latest version when immediately changed. As for the update on your Fire TV Cube, it's not possible for us to know why it updated or needed to be updated. Every Fire TV Cube is going to be mostly the same. However, location and other factors can prompt updates sooner or later for certain devices.

So basically, just forget about verifying update numbers, cross your fingers, and hope for the best, right?