It doesn't help that T-90 itself has more variety in its 20 years of existence than any other tank. For example the "top" answer here >>559078 ignores the fact that T-90 entered service with a cast turret and Kontakt 5 armor, whereas is now using a welded turret (massively improved passive armor) and Kaktus+Arena.
Comparing the two makes sense, but saying they're the same or acting as if there's no difference between the two is dumb. T-90 has a newly invented engine, a newly invented FCS, a newly invented cannon (based on T-80), new passive armor, and new energetic armor. It also included as built-in the concept of active protection systems pioneered way back in the 80s. Every nut and bolt is different, even the tracks used a different design that allowed them to be lighter and easier to repair, they aren't interchangeable. The main hull was modified to expand carried fuel, make it safer for the crew, it's about a meter longer than the T-72. The construction methods were also changed, instead of doing casting they did welding, which brought costs down (among other things).