Disclaimer: I'm not a dark-mage-of-the-smith-chart - I'm just a dude who had to take a course in EM once. Honestly, I'm pretty ignorant and you should take all of this with a grain of salt.

My understanding is that the noise model is very different - that LTE/ofdm are very good at getting around radio noise by-frequency - ie a bunch of interference at a particular frequency. But 3G/umts/cdma was very good at dealing with multi-path distortion because of the whole chip-code-orthogonality property and the fact that the radio would have multiple time-shifted taps on the antenna. That would mean that in theory LTE would be better at dealing with persistent environmental noise, but 3G would be better in a city where the phone is receiving that same signal but reflected against buildings and slightly time-shifted.

I doubt this makes any difference in your case - it's probably just that 3G is typically on 850MHz and that LTE is probably on a higher frequency like 2300MHz which wouldn't propagate as well.