After thinking about it a bit more, the only reasonable conclusion I can come to is the following:

It's the number of times users actually typed

    bower install lodash

i.e. adding it to a project that didn't already have it, as opposed to just running bower install on a package which uses lodash.

Would also explain why despite bower itself being installed 37000 times in the last 7 days, the most installed package is only listed as 100 or so times.