Walk-up tickets between Liverpool and Manchester are so cheap I'm not sure why you'd need to bother with these! It's not like Leeds - York where there are no other advances and a stranglehold on Off-Peak fares.
By the way gray1404 EMT Advances are sometimes cheaper when booked through their own site. Be careful what you wish for from EMT if it's an
EMIDSTRAINS.ONLY walk-up you want.... (OK I think I can guess their intention there, because there appears to be a negative easement on Lincoln - London via Nottingham, but that was the wrong way to approach it if that was what they wanted to do! Talk about cowboys.)
The reason availability is so much greater on Northern than it is on EMT and especially TPE is because the Northern services don't go beyond Manchester (to anywhere except Manchester Airport) so you are only competing against people on thinks like Broad Green - Manchester or Irlam - Liverpool Advances (and lets face it, people who live in those places probably have no idea they can get an Advance yet). On the long-distance services you are competing for quota with people joining the train at Liverpool and every station before Manchester who is then going beyond Manchester. e.g. Warrington - Sheffield people will take up quota that you need between Warrington and Manchester. OK that particular flow is probably not that busy but I think you get the point as it goes for everyone going from Liverpool to east of Manchester too.
Lastly I should imagine that TPE particularly want to be careful about the quota they are offering to Advance buyers between Manchester and Liverpool as they are most likely to be targeted by people booking tickets at the station on the day for offering the fastest services - although it has always perplexed me why so many people wait to board westbound EMT services at Piccadilly and Oxford Road when the Northern service is quicker (and now 319-operated... OK so I guess no more tables
But much less risk of pacerification).