Variation 11 features some striking reversals of the metric structure used so far. Vivaldi begins the variation with a half-quarter rhythmic pattern which correlates to our expectations of meter. In bar 5, the bass reverses this pattern. This small change is significant in our perception of the music and contributes to the rhythmic variety of the piece.
If variation 11 looks simpler than it is, the opposite is the case in variation 12. The switch to a 9/8 meter serves to accommodate a series of three note diminutions in the upper voices (we are not dealing with those at the moment). The change is in notation only, from a simple triple to a compound triple meter. The way we perceive this rhythm is as simple as before.
The actual diminution in the bass here is minimal (suggesting there will be extensive diminution in the upper voices)