The honest answer to this one is always that any range over very short is often down to luck. The real physics is simply what’s called path loss. So anything in-between the two antennas reduces signal. Once you get to the fringes, it gets very unpredictable. You can have two people with their radios up to their mouths talking perfectly well. If one puts the radio back in their pocket, the other can no longer call them - just the short distance between up and in the clear and 180 degrees of the signal blocked by the body wrecks it. Lots of people calculate range from the ‘operating position’. I always feel it should be from the stowed position, because that’s where it is when people call you!
Top of a ridge gives you excellent coverage down into a valley - and the range increases vastly! Pop over the ridge just a few metres and it’s gone. Portable operation can be increased greatly by rigging up antennas that are in more sensible locations. The Police in London back in the 80s onwards had their microphones clipped to their lapels, so they could turn their head, press the button and speak without holding the mic, with the radio on the belt. Range was pretty limited until they started putting the antenna on the microphone, where it poked up above the shoulder - and range and reliability went up greatly. The military do the same - the radio may well be in the webbing or in a back pack, but the antenna goes up high.
People always assume that extra power cures all this, and while at the very fringes, it might, in general, lifting a radio up above your head improves the range more than multiplying the power output by ten!