A little simplistic, but one reason why EE (orange/t-mobile) had double the masts to O2 or Vodafone.
The 4G rollout has changed everything for all four networks (EE, Three, Vodafone and O2) - the old rules no longer apply. 4G also travels further than 3G. (simplistic).
Well here in rural Oxfordshire, there's no 4G in our village. I can only get a signal on O2 or Vodaphone, despite being 1Km away from the nearest mast, sometimes not even on 3G.
It's a basic fact that higher frequencies suffer higher atenuation, at least in the GHz range. Just for example, take a wifi router that operates with 2.4GHz and 5GHz. You will find the range of the 5GHz signal much lower than the 2.4GHz one.