Difference between two antenna types
I don't have a full answer (and it will be hard to get one without a lot more measurements with instruments you maybe don't have -- antenna design is a black art even with good tools.) But at the distances you're considering, near field effects are likely present. So the loop may be picking something up inductively, which the dipole cannot. Inductive coupling will be much less sensitive to the exact length of wire involved -- tuning an antenna to resonance without tools, by just cutting to length, is unlikely to work very well except by luck, so your antennas are probably not that close to being resonant.