Eventually you will start to get invalid/rejected shares, it’s part of mining. You have to feel comfortable with your farm efficiency and then just leave it at that. There are too many variables and there are things that you just can’t prevent. You can submit a share to a block that has already been solved. This isn’t ineffciency on the card, it’s just bad timing. If you keep resetting the miner for a few bad shares your’re going to have a bad time.
Hopping from one pool to another is also inefficient. The payout plans are designed to discourage pool hopping.
In the previous screencap your farm efficiency was at 99.76%. That was a really good ratio.
Here’s a screencap from a random miner on the ethermine pool. They have 2% rejection.