T-95 was a tank program to replace the T-90/72 family of tanks... much the same way the PAK FA was to replace the Flankers and Fulcrums.
The problem was that the conflict in Chechnia made them realise that mobility was important, as was protection and firepower, and that sometimes lighter, more mobile vehicles were needed and at other times heavy well protected vehicles were needed.
This led to a change in thinking... instead of making one MBT (ie T-95) and one IFV (BMP-4) and one APC (BTR-82) and one light armoured vehicle (BRDM-4), that for each operational brigade you need all the vehicles to have similar levels of mobility and similar levels of protection, so they all should arrive together, and they should all be able to withstand similar levels of enemy fire.
It means the MBTs wont be left on their own after the enemy have picked off all the light vehicles like BMPs and BTRs.
The solution was a completely clean sheet of paper.
There was only one T-95 and it was a main battle tank with a rear mounted engine.
Armata is a vehicle family that includes vehicles with the engine at the back, a turret in the middle and crew in the front like the MBT, but it also includes IFVs and APCs which likely have front mounted engines so the troops can exit from rear ramps.
A lot of things that were developed for T-95 will have been transfered directly to the T-99 (ie Armata MBT)... but the point is that there will likely be four T-99s... T-99h for heavy, T-99mw for medium wheeled, T-99mt for medium tracked and T-99l for light... the heavy model will be tracked and the light model will be wheeled, but they will all share electronics and sensors and weapons.
The T-99l might have a 45/57mm gun because it is simply too light for a 125mm gun for instance but it will have Kornet-EM missiles to allow it to engage enemy MBTs if they stumble on them in combat.
The brigade structure with the four vehicle types was theoretical and was pretty much passed to the Russian MIC when the T-95 was cancelled so they all started from scratch then, but the companies these design requirements were passed to started from scratch in terms of layout, but they could use existing things where useful... not everything needed to be made from scratch.