From my limited experience, I absolutely agree .
My work iPhone 5 on Orange has abysmal signal in the majority of places. Drop in my personal EE sim and magically I get substantially better signal both in terms of voice and data.
My wife is on Virgin Mobile with ok signal but then in goes a new payg EE sim and signal improves as if I had stood close to a cell tower.
I moved to EE from O2 who were a lot worse in my experience, on O2 I had minimal data and terrible voice (quality and failing calls) and also the dreaded 'no signal' even in London. Moved to EE and the signal is signifcantly better.
I'm not entirely sure where Rootmetrics test phones travelled to but I'd suggest that they missed the majority of the countryside areas - the Derbyshire dales, the lakes, Surrey hills, South Downs national park to name a few. Maybe they consider heathrow as rural these days?