I've done dozens of TV installations over the years... and I've had to go to Home Depot dozens of times over the years to get the damn screws I needed for the TV.
The problem is NOT with your mount, it is entirely that TV manufacturers don't package, by default, the exact screws in the box that you need to mount their TV. It's not that difficult of a concept, but at the end of the day, TVs have backs which are all over the place. Some TVs I've used have needed extenders to get the mount to sit on the back flat. Others needed shorter screws than what I had available.
At this point, I have hundreds of screws of different thread sizes and different lengths, so I feel relatively confident that I'm covered with most TVs I buy. But, I'm still always ready, and rarely surprised, if I need to make that run over to Home Depot to get exactly what I need for any TV.
Don't let it bug you or worry you in the least. As long as you are within (or nearly within) the weight specifications of the mount, then you will be good to go.