I can imagine if I was a sahm I would say this but probably add because DH works away. To feel the need to explain shouldn't be required but I guess it's to avoid the possibility of judgement.
As with anything it comes down to stereotypes; DP earns a lot so Mum can live a life of luxury; DP doesn't earn much so rely on benefits to top up with several kids.
In reality it's just a case of personal decisions and no one else's business. When anyone tells me they're a sahp I just think great, must be hard work with toddlers and babies, even when school age I don't really think about it, each family has their own reasons.
I did encounter one strange experience many years ago. A group of us teacher colleagues went to have pre drinks at a friend of theirs house. Huge, gorgeous home in an expensive area, walking distance to local pub. I didn't know her and just casually asked what do you do? She replied I have kids which keep you in a job. Yes, just that! I was taken aback, think I said something like oh well we definitely need kids to be able to teach.
Maybe she was annoyed people ask, was in a bad mood or just a rude person, I don't know. I was quite young, as was she, it really was a getting to know you question.
Since then I'm cautious about asking so wait until it's obvious to chat about work stuff.