Why is a Tank that entered Service in 80s, using Shells from the 70s?

It’s relevant to capable ammunition being chosen for specific BRs.
An aimed BR is chosen for the frame of the vehicle above its minimum viable ammunition, then further ammunition is added to fulfill that BR, or in some cases added after the fact if the initial ammunition wasn’t enough.

This is just a pattern I noticed over time, most notably we see 2A5 and Challenger Mk3 go through the added ammunition after the fact for further BR increases.