[kbenson]

I imagine for some instances, it's because a security fix might have gone in, but they don't want to clue attackers on other platforms. For example, if the fix is submitted for the Android and iOS apps to their respective marketplaces for review at the same time, and one is authorized and goes live first, then you've released information that's useful to someone looking to exploit the other (they now know there's something to look for in the differences of how they operate).

That is, unless both stores allow you to get the version authorized but hold it for release until you want. I know the Roku store did that years ago, so I imagine it's probably a feature that's present in those stores, but I don't know for sure.

[2xlbuds]

That's another great reason to be vague, because even if you were able to launch your updates at the same time on all platforms, at least on mobile the adoption rate can be relatively slow - on mobile I've measured that it takes about a week to get 80% of users to update and 2 weeks for 90%, which could leave a sizable vulnerable population.

[saagarjha]

Both the Google Play Store and Apple's App Store have this feature, I believe.