Sprint version of Samsung Galaxy S4 already gets an OTA update
By the way, the initial version of the software is L720VPUAMDC. If we might be allowed to speculate for a moment, the only difference in the old software version and the new one is that the last letter has been changed from a "C" to an "L". Perhaps the update flipped the LTE radio on inside the phone, which had previously been set up to work on Cellular only.
You can always try to coax out the update manually by going to Settings > More > System Update. This is not an update to the rumored Android 4.3 build that is now talked about as being displayed at Google I/O so again, we wouldn't lose any sleep over it.