Certain? Hardly - again, first look at variance. Second, see the hedging of -ish. Just as has been used throughout this thread by devs and others -- -ish is a hedge to mean could be in the vicinity of. Not binding. Not exactly. Not certain. Not by image, not by perception, not by actualization.
Second, see the first part of the entire parenthetical you quoted out of context (colored for ease of recognition):
(the above is a guesstimate - I don't believe the devs have given an estimate --- but this is realistic)
The "but this is realistic" means using common sense - not some half-wit, cock-in-hand coping nor some rigid algorithm.
Hardly - there is nothing that leads to your hypothetical assumption with Sifa, nothing at all --- where as we do have some trends with Ashley, as well as past trends with other family members.
I think you're splitting hairs for some odd reason - when it's obviously a well-reasoned guess - yet, still a guess -- and if you want to continue to do so, might be best to move this to off-topic.