The Apps are just confusing me a bit, can someone help?

Question

There is 3 types of apps available; ones that work on iPhone, ones that work on iPad and ones that work on both. But the thing that is confusing me here is that when a developer makes an app for both devices I would have thought that they would make them under the same app, (like the ones that have a + next to them on the app store). But now I have found apps that are the same (one for iPad, one for iPhone), but don't come under the same app on the app store. Now this is what confuses me because as far as I understand if I want the same app for my iPhone and my iPad I will have to buy it twice, once on my iPhone and once on my iPad, even though it's the same app. Now maybe the developers do this on purpose to make more money, but it makes it more difficult for me to organise my apps on iTunes because I would have 2 of the same app and I would have to organise it so the right app syncs with the iPad and the right app syncs with the iPhone.
Have I got this all right or can anyone correct me if I'm wrong?

Answer 1

Well, yes in the sense that they provide a higher screen resolution because of what an iPad is capable of displaying. Mostly, though, I think developers just use the abbreviation to distinguish an iPad app from an iPhone/iPod app. In reality everything on an iPad is "HD" if you think about it. Nothing like a little marketing hype, though.
The iPad's display resolution apparently conforms with the HD resolution on a TV when displaying 480p. Of course that's not as high a resolution as what many TVs can display for HD. From what I've read to display 720p or 1080p you require a higher resolution than can be provided by the iPad. I think for 720p you need a screen resolution of 1368 x 768 and for 1080p you need 1920 x 1080. iPads are 1024 x 640.