The difference is
Climb/Descend FL...
Climb/Descend TO Altitude ...'
the to is only used when referring to Altitudes, as one of the ways of picking it out, particularly in the descent when you read back the QNH as well so you remember to set it and don't go busting a level or MSA.
Perhaps the climb and maintain has crept in where controllers are trying to tell you that is your last expected cruise level, but it has then become generalised for all climb instructions.
Often non-standard phrases will creep in regarding your final requested level.