Let us consider that read_count = 7, means there are 7 readers processes are there in the critical section and they want to leave it.
In assembly language, $\textbf{read_count --}$ would be implemented as
1. READ temp, read_count
2. SUB temp, 1
3. STORE read_count, temp
Consider that 3 processes want to leave the critical section at the same time and when $\textbf{wait(mutex)}$ statement is not there then all leaving processes read statement 1. and wait preempt for short duration, then all 3 decrement temp value and store it to read_count and at the end read_count will be 6 instead of 4. Which means readers are still present in the critical section even though they are not. So in that case writer may not even get the chance to enter into the critical section or there are no readers in the critical section but still read_Count will show some reader processes are there in the critical section.