Paul Corfield writes:

Hounslow got H20 up to H37 plus oddities like the H98.

The H91 follows the 391 route most of the way between Hammersmith and
the Chiswick Roundabout, which has always seemed to me to be more than
just coincidence. The other oddities may have similar similarities,
or maybe Hounslow just ran out of numbers and took 90-99 as well.