Good question. I don't know for sure, but can offer a couple of possible explainations:

1) They are temporariliy dropping out of sleep paralysis

2) Sleep paralysis is not an absolute thing for some people. They have limited paralysys. Take the case of sleep walking for example. Not much paralysis there.

Everyone is different and located somewhere on the curve. Some experience total and deep paralysis, some experience almost none. The rest of us are somewhere in the middle.