If you really want to know why and how this happens, without all the techie speak - - - -
Verizon's 3G data and voice network is CDMA, and only a device with SVDO technology can use voice and data at the same time (as opposed to the current standard EVDO) on CDMA. The first device to do this was the HTC Thunderbolt. The Galaxy S3 also has SVDO, so you can surf and talk at the same time if you have Verizon 3G service.
The VZW iPhone does not have SVDO, so it can't do it. The iPhone on ATT and Tmobile can do this, as those two carriers don't use CDMA - they use GSM, which can do voice and data already. I don't know why specifically, but just know that Tmo and ATT are GSM and Sprint and VZW are CDMA.
Finally, any 4G LTE enabled phone on verizon can do talk and data at the same time, as the data will run over the LTE network and the voice runs over CDMA. So, devices like the Moto Droid Razr and other LTE phones can do talk and data at the same time, but only if LTE service is available. If in a 3G-only area, the Droid Razr can't do talk and surf together.
Make sense?