PyGrinder block and sequence missing algorithms are not reaching the correct percentage of missing values

[giacomoguiduzzi]

Issue description

Greetings,

I'm working on a project related to forecasting time series with Deep Learning methods. A quick question about sequence missing and block missing from PyGrinder: I noticed that when I set a replace_pct value of 0.5 I am not actually getting around 50% of missing values, but 39%. If I raise this value to 0.75 then I get around 50%. Is this normal? Am I missing something?
Let me know if there is any additional information I can give you regarding this behaviour.
Thanks in advance, I'm looking forward to your kind response.

Best Regards,
Giacomo Guiduzzi