According to bower stats over the last 7 days lodash was installed 23 times compared to 4 million npm downloads & 181 new npm packages depending on it.
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.