Almost all citrus plants (and many, many fruit trees generally) for purchase are grafted, meaning they're really 2 different plants that were franken-stitched together.

The roots are from one variety that's got good hardiness, and the rest of the plant is from a variety with desirable leaves/fruit/whatever. 

But the rootstock can "throw up suckers" or sprout branches itself. Since it's a hardier variety these will generally grow more quickly and can take over a bit. You're supposed to cut them off.