Stair riser height is not uniform

First post here. I am hoping someone can help me. I posted this in the flooring forum as well, because I was not sure which one would generate the best responses.

This past weekend, we had 2000SqFt of hardwood flooring installed in our two story home. The floors look beautiful, everything is trimmed in nice, however, our stairs are jacked.

Background detail:

The bottom landing of stairs was tile, which was resting on 1/4" thick plywood, on top of subfloor. Assuming tile is 1/4" thick, the total height from subfloor, to the top of the tile is 1/2"

The top landing of the stairs was carpet. No additional plywood, other than the carpet mat. It was a thicker type of carpet, but anywho.

With the hardwood floor install on the stairs, I requested that the installers remove the treads and the risers to my steps, and install the treads and risers as single plank stairs in the hardwood. They did this, however, my stairs are now completely out of wack.

The first riser from the bottom is 7 1/4" high. The next 11 risers are 8 1/4" high. The last riser is 9 1/4" high. Obviously, this goes against everything that is right for safety - with a pregnant wife and a 3 year old child, I have to get this fixed.

My hardwood floor company is stating it is impossible to fix and that my stairs must have had a bad stringer installed with the stairs were done in 1974. I measured the stringers from the basement (looking up) and the riser height was uniform across the board. Also, the stairs never had that obvious look of being 'off', as they do now.

Any suggestions on how this issue can be fixed? I know modifications of the stringer is probably needed?