The artist just uploads stuff whenever on twitter - so technically the chapter numbers are the number they came out of just some of them haven't been translated until recently which is why we have sporadic uploads with seemingly random numbers.
I mean men can still have kids pretty late, so it's not impossible that grandpa had a new kid (possibly out of wedlock) and then to hide the shame they gave it to their son and told him to raise it alongside their son.
Though depending on the grandparent ages, if they are "relatively" young...
Eh, I've done this myself in the past, taking someone else's shift for them - but I got paid the full amount and he didn't suffer any penalties because he found his own replacement for the day.
You have to remember he's also being an ACTUAL second for the boxing gym he owns and pays for. Whenever there's a match for his own fighters, I think he tends to end up making less pages.