How has Sanders pulled even with Buttigieg? By doing extremely well at satellite caucuses, or the alternative caucus sites for people who couldn’t make a regular caucus (e.g., people who live out of state, or locals who simply couldn’t go on Monday evening). Satellite caucuses are unique among caucus sites because they aren’t worth a set number of state delegate equivalents; instead, each satellite caucus’s state-delegate-equivalent value is determined by how many people attended it.
A savvy campaign might have realized the potential to run up its state-delegate-equivalent score by encouraging its supporters to attend satellite caucuses, and that seems to be what the Sanders campaign did; according to The Intercept, it devoted a lot of effort to getting out the vote at satellite sites, while no other campaign paid the satellites much heed. Apparently, it paid off: So far, Sanders has gotten 21.855 state delegate equivalents out of the satellite caucus sites, and Buttigieg has gotten 1.196.