I have a 2 switch fixture on the wall, top controls lights over sink, bottom controls exhaust fan. The top switch was starting to act up so I went to Lowe's and bought the same identical switch that I have on the wall. I wired up the new one exactly like the one that was already on the wall. I turn the top switch on and BOTH the fan and lights come on. I am stumped, it's 3 wires, I marked everything to make sure they all went on the right screws on the new fixture. What the heck is going on?? I'm using a Leviton fixture too.

The hot wire goes to the side of the switch that connects together by a small tab. The other side has 2 isolated contacts and need to go to light and fan.
You could probably disconnect wires, turn switch upside down, reconnect and try, you can't hurt anything.
Main thing is hot to the side with a jumping tab by screw.

Check the old switch, look at the Line side, see if there is a jumper missing. To know what one looks like, look at your new switch. On one side you will see a small piece of brass joining the two terminals. If you see this missing on your old switch, the jumper on the new switch needs to be removed.