It's an interesting topic, and I've found the same thing. Obviously some solos are more difficult than others. But I often react very differently to solos of roughly the same intrinsic difficulty. Like I mastered the solo to 'Let it Be' in a few hours, while it took me ages to suss out 'Rock me Baby'. I haven't got a clue why that should be - they're both short, relatively simple, and I've heard (and loved) them being played hundreds of times. So I'd like to know more about the complex relationship between the song and the guitarist - maybe someone on GT can explain, although it sounds like a subject for a music PhD! At the same time, I think it's easier to understand the other phenomenon, where a guitarist's performance of a particular solo varies from one day to another - it's probably just down to down to tiredness, stress, distraction, the kids, and (in my case) shouts from downstairs that I need to quit practising and get to the store before it closes!
Best wishes,
Steve
