There is no reason for stating why there are differences between a 2020 iPad Pro and one from 4-5 years, ago!
It should be pretty obvious why.
The design of the iPad Pro and the new 2020 iPad Air 4 has changed.
The newer 2018/2020 iPad Pros are smaller overall because they have much smaller/thinner outer bezels.
The older iPad Pro models from 2015-2017 measure overall 14.2 inches on the diagonal with a 12.9 inch screen, also measuresd on the diagonal
This is because the outer bezels of these older iPad Pro models are much thicker/wider and the needed room wqs to incorporate the older Home Button/Touch ID sensor.
The newer 2018/2020 models have smaller thinner outer bezels, but the diagonal 12.9 inch active screen size area is the exact same, but the overall diagonal size of the iPad is smaller now due to smaller, thinner bezels.
Also, the new iPad Pro models have NO Home Button, any longer!
New iPad Pro models use FaceID and NO LONGER use finger, Touch ID/Home button that was on older iPad models.
If you still have both the older and newer iPad Pro models, it should be obvious the difference is size and feature differences.