Ann: Appen 2018 Full Year Results, page-59

I can think of two reasons the multiples for ALU are so much more significant than APX. 

1. ALU is recurring, subscription based revenue, as mpacman said, and it's sticky. Whereas APX is contract, with a majority of revenue coming from a few large players (think google & Facebook). Though apx seem to be doing a good job of penetrating more markets and diversifying away the risk of losing one major contract, this risk is still there (however unlikely).

2. ALU continue to provide longer term growth targets, whereas apx only gives guidance on the upcoming fy or hy. Although ALU's targets are only ' aspirational' and not actually guidance. At the moment it looks like they are going to sail through their 2020 target with ease.

Now don't get me wrong, growing eps from 14c to 38c in a year is a remarkable achievement and i love my apx holding (bought 10K at $2.9, have since taken out ~ $15ks worth at $6 then $12 and still have 1000 units!!!!!! What a winner!). But, in my opinion, that is why ALU has consistently traded at a higher multiple (apart from Mr market not always being rational)