That was my thought too. My tickets are always Manchester Stations to London because I use Metrolink to connect. I imagine some people buy advanced tickets from Liverpool to London and then buy a flexible return on the Merseyrail so those tickets will still be classed as Liverpool Stations ticket.
The other variable is that Liverpool is located on the coast and therefore its catchment area is smaller than Manchester's. Parts of the Wirral will have an easier service via Chester and for the eastern areas of Merseyside, Wigan and Warrington may be more attractive. Most of Great Manchester's London pax will travel via Piccadilly or Stockport. The exception is Wigan and maybe parts of Bolton.
I'd expect that people from parts of West Yorkshire, Derbyshire, and Lancashire travel via Manchester too (e.g. Huddersfield, Buxton, Chorley). Outside of Merseyside, I'd expect that very few people travel via Runcorn or Liverpool. Purely comparing the populations of Greater Manchester excluding Wigan (2.5million) and all of Merseyside (1.5 million), you can see that Manchester has a far bigger catchment area.
Before everyone from Liverpool piles in on me I'm completely on board with the 2nd service per hour and think its justified. Liverpool is never going to be as big a market as Manchester though purely because of its position on the coast.