XCode iOS Base SDK setting question

I have an app that I build using XCode 3.2.6, Base SDK 4.3, Deployment target iOS 4.0. I decided that I should test it with the latest iOS5 beta, to make sure that my app doesn't break, and submit any bug reports to Apple if I find something.

What I found is that if I build the app using XCode 4.2, Base SDK 5.0, Deployment target iOS 4.0, the size of the text in my UILabels and UITextViews renders smaller (by maybe 3 points or so). But that's not really what's important.

I have two test devices, one has iOS 4.3 installed on it and the other has iOS5b3 installed on it. What confuses me is that the shrunken text behavior occurs on BOTH devices.

If I build the app (like I normally do) using XCod开发者_如何学JAVAe 3.2.6, Base SDK 4.3, Deployment target iOS 4.0 the shrunken text behavior goes away on BOTH devices.

My understanding was that the UIKit libraries were linked at runtime on the actual device, and as long as I use APIs that are supported by both Base SDK versions it shouldn't matter which Base SDK I choose.

It wouldn't be so surprising to me if I observed different behaviors on devices running different versions of the OS. But as far as my understanding goes, choosing a different Base SDK shouldn't change any behaviors on a given OS as long as (of course) the Deployment Target setting is low enough to include the OS version and I'm using APIs that are supported.

So what gives? I'm obviously missing something.

I really appreciate you taking the time to help me understand this. Thanks!