Having just done the Part 2 Written, I would say a few factors would contribute to the skew in results:
1. If you haven't developed good exam technique and aren't prepared for MRCP questions in general, you have a good chance of failing Part 1 anyway. The population sitting Part 2 by definition have to have passed Part 1 so there is that difference in demographic that contributes.
2. The questions in Part 2 are more clinically orientated and you're more likely to perform well based on accrued clinical experience and real-life medical knowledge as opposed to the much purer theoretical and scientific knowledge base required for Part 1.
3. If you've recently passed Part 1, you're very likely to still have a lot of that raw information well established in your head; though Part 2 is more clinical and diagnostic, there is significant overlap in terms of the underpinning knowledge. I think candidates build on top of the previous examination revision as opposed to starting from scratch.
4. Some people who aren't committed to doing MRCP(UK) or a medical specialty will sit Part 1 Written but go no further, more of these people are likely to fail relative to the people who commit to doing Part 2
5. I'm fairly sure the Royal Colleges have the score scaling set up with the intention that more people should fail Part 1; this is purely speculation (though I know various consultant colleagues involved to lesser or greater degrees in the RCP who think the same) but I think it's probably intended as a hurdle to filter out candidates in a front-loaded manner.
I'm not convinced the questions are
easier (in fact they're still very hard) but they tend to be more clinically orientated as identifying a diagnosis, how you'd investigate or treat something, what the prognosis of something is, what the most likely underlying aetiology is et cetera. If you've been doing medical specialties for a while and are quite experienced (and enthusiastic beyond 9-5 grinding) it can feel easier than Part 1 as a result. There's no reason you couldn't do this in an 'FY3' year but depending on the level of experience you've had so far in what types of departments and what amount of responsibility, it could be quite challenging to approach compared to the sheer knowledge requirements of Part 1 which are best learnt by learning and MCQs.
Since I didn't gloat earlier, I got 724 in my Part 2W