It adds 3 and subtracts 1 instead of adding just 1

Quartus will issue the sensitivity list warning whenever you read the value of an externally declared signal within a process UNLESS the read occurs under a clock-edge. In your case, you're reading "count" when you say count <= count. This read occurs in the else branch, so it's outside the clock edge. Prior posts indicated the correct recommendation - just avoid the unnecessary else branch.