Question about fermentability and attenuation

As I read about going all grain, there is a lot of discussion regarding the fermentability of wort and the fixed fermentability of extract. Then there is also the ability of yeast to attenuate. I just want to make sure I have it all straight. For example if an extract is fixed at 75% fermentability and you get say 77% attenuation, does that mean that of the 75% of the sugars available to be fermented, the yeast successfully got to 77% of those? So if you brewed the same beer as an all grain version and mashed lower and got a more fermentable wort, the same 77% attenuation could actually "eat" more sugar? Thanks for the help.

 

I'll let some of the more senior members chime in too but the way I understand it you are correct.

 

Thanks for the replies. It may seem like a simple question, but I just wanted to make sure I was thinking of it correctly.