The protocol question is a good one. At the present time, none of the Z wave lightbulbs are rated for outdoor use, but some of the zigbee bulbs are.
Also, there are zigbee bulbs available in the BR30 and PAR38 “flood light” form factor, while the Z wave bulbs are still available only as A19’s or A21’s, the “regular” indoor shape. And in the existing offerings the floodlights are also significantly brighter.
Additionally, while it is true that Z wave plus has a longer range, for technical reasons zigbee is somewhat better at transmitting through humid conditions, including rain and snow. (This is detailed in the various “zwave versus Zigbee” forum topics.)
So you put all that together, and most people use zigbee for outdoor lightbulbs.
As for why any particular repeater placement works one while another one doesn’t, it’s usually just local architecture. Both signals are Omnidirectional, and some surfaces are reflective while others block signal. The signal might be bouncing up a floor and out a window down to the other device. Or it might be taking the long way round and it happens that in the successful location the message is actually going backwards over to another repeater which is then sending it back forward to the eventual target.
Smartthings doesn’t give us network mapping utilities, so we can’t see any of the pathways. But as long as the message is getting through, that’s what matters.