The good performance you’ve observed could be because helical antennas are very good at going through obstructions that would otherwise cause trouble. I have no experience with them in radios (unless they’re buried in some of my FRS/GMRS radios), but I’ve used them quite a bit for 2.4ghz wifi.
I assume the 2.5" you measured is the length of the coiled segment? Remember that the antenna length is the actual length of the wire if you were to stretch it out straight.