Cannot limit ads insertion

My setting is to insert the ad block for every 300 words in a page. So I have 3 page with different length of content:

– Page 1: 1k words
– Page 2: 7k words
– Page 3: 34k words

Then I have the limit for max ad insertion: 10

Please have a look at my setting in this image: https://imgur.com/kfdJsRG

Then I notice this when debug:

– Page 1: First block start with the count: C=2 N=4, then the limit works.
– Page 2: First block start with the count: C=2 N=23, then if I apply the limit, the block is not appear
– Page 3: First block start with the count: C=2 N=105, same as above

So, how can I limit the ad insertion correctly?

Thank you.