I thought the only battery the JB took was a 18650??
That's correct, but as mentioned above the 18650 is available in different "mAh" ratings, or "milli-amp hour". Here's a loose analogy I grabbed from a quick Google:
The milli-amp hour is the standard unit of storage capacity for a cell.
It is similar to "gallons of fuel" for an internal combustion engine. The milli-amp hour rating of a cell tells how many constant milli-amps of current can be supplied by the pack for one hour. This rating can be used to find the duration that a battery pack can provide given a certain draw.
Because cells are wired in series, the milli-amp hour rating of a pack is the same as the milli-amp hour rating of a single cell.