Why? If I choose to cheat on a test to pass a class and avoid studying it's a poor decision (from a risk vs reward perspective) and also a dishonest one (from a societal perspective). Many dishonest choices are poor ones.

I guess you're arguing that poor implies bad judgement which is a lesser crime than dishonesty? But actually in some cases I'd rather have the latter than the former.

Poor is deliberately chosen in an effort to understate things.

I can see why she would try to downplay her actions and hope that everyone else would play along.

Here's how I interpret the two phrases:

"I done fucked up" vs "I lied"

Personally I don't really read it as downplaying (though I accept I may not attach a normal level of stigma to dishonesty)

I specifically chose those words for their generous effect.