Today I was on a 21 in the Old Kent Road and on the approach to each
bus stop, both the display and the voice announced the same thing, eg
"Trafalgar Avenue".

But when the bus was ready to leave each stop, the display announced
"21 to Lewisham Centre" but the voice announced simply "Crystal
Palace".

This was more like the way that the next stop is announced, although
it corresponded to announcing the route and destination.

Maybe the voice says "Crystal Palace" every time it doesn't know what
to say?

It reminds me of the SWT 450s which, while the display was showing
"all seats on this service are standard class" (or something like
that), the voice ominously announced "One".