This is interesting Richard as a “direct” comparison of locations. I would say however that your mini-loop in the garden would be screened by the surrounding buildings and hence it is not a fair comparison.
Normally a home station antenna is better than we can manage to take with us to a summit and would be at least partially clear of surrounding buildings.
I think the lower noise level from the portable site is probably as important as the clear way for the signal to get out. Were you only able do transmt comparisons or did you mange receive ones as well? The latter is difficult unless two people are doing the tests at the same time at the two sites because of propagation variations.
There are several SOTA chasers who can ONLY chase from portable locations (because of restrictions or interference at the home QTH) and their signals are often some of the stronger ones on the bands!