It takes me about 10 runs to get a line of output without the grep. Sometimes less. Try it :)

EDIT:

test.sh:

  for i in {1..1000}
  do
  ps | grep grep | wc -l
  done

./test.sh | sort | uniq -c

  13    0
  987   1

Which would indeed suggest 1.3%. Likely dependent on how quickly you're running it, if it's in memory already etc etc