This is described somewhat in the answers here: Naming Scale Degrees
Basically, scale degrees are typically numbered according to the (parallel) major key, even if you're actually playing in a minor key, or some other mode. Thus in your case, A major would have a G# and an F#, so the bVII and bVI tells us that they have to be lowered (the sharps removed).
It may seem confusing for it to reference the major scale, but it actually avoids potential ambiguities, in that when you see a VI (for example) you don't have to stop and think about whether the 6th scale degree in your current mode is an F or an F# -- the former is always a b6, and the later, just a plain 6. It might be helpful to think of the flat symbol, in this context, as simply indicating a minor interval, rather than an actual scale alteration.