The number of evaluation pool decreases in some generations

Hello Everyone, I've a question concerning the number of evaluation pool.

Now I set the population number to 30 and generation to 7.

As It's SPEA2 the evaluation pool should have an population set of 30 and another external population of 30, so the total number is 60 solutions.  but one of these middle generations' evaluation pool is 47. In a similar case it reaches 61.  

That drives me crazy, anyone knows why the number decreases or increases !! 

Thanks in advance