Why is 'seven months' so interesting?

So, according to this show, a female gibbon gestates for seven months.

Why did the Major think this is interesting enough to mention to Basil, and why does Basil also react to this information as if it's really something to think about?

I mean, what's so interesting about seven months?

I could understand it, if they gestated for twenty months or just a couple of weeks - it would seem quite remarkable. But seven months is close to the nine/ten months of human gestation (the latter is based on moon calendar and the moment of conception, rather than the western calendar, etc).

So why is it so remarkable that two adult men are almost mesmerized by the number?

Is there some kind of hidden meaning that only british people can understand, or is it some kind of inside joke?