There are many reasons that the iPhone's OS is consistently more fluid (and still is). My wife's iPhone 4s has a more consistent UI throughout than my Galaxy S4 GPe. I'll focus on a few quick reasons for this, but there are several more, some of which were touched upon already. But they can be summed up in two terms; higher specs on the iPhone and lower overhead on the iPhone.
Some people don't want to hear that the iPhone is higher specced than their Android device of choice. However, these people would be well served to look at the history of Intel's Core 2 Quad Q6600 and the Core 2 Duo E8400. On paper, the Q6600 was more powerful. However, it couldn't reach the same clock speeds as the dual-core E8400. Since most applications are not quad-threaded, the E8400 was (and still is) noticeably faster in most games.
We're seeing a similar approach with the Moto X. The Moto X and HTC One essentially have the same CPU and GPU combo, except that the Moto X uses a dual-core CPU. And yet, the X outpaces the HTC One in most benchmarks (except the most demanding synthetic CPU benchmarks). That is likely due to thermal throttling. The combination of too much heat (small form factor) and no real need for those four cores leads the system to throttle down to a lower speed than the Moto X, leading to lower effective performance.
Apple has always had a capable CPU paired with the best GPU (usually a generation ahead of what's on Android). On top of that, Apple had a heavily GPU accelerated UI before Android did. Lastly, Apple had lower overhead.
Take for instance, the home screen. In early 2010, it was the Nexus One versus the iPhone 3Gs. The Nexus One had to account for at least three active screens (application drawer, notification shade, and home screen) at 800x480 resolution, while the iPhone 3Gs had just a 320x240 home screen. Assuming 24-bit color, 8-bit transparency, 60fps, and nothing fancy, we're talking about a fill-rate requirement of 276.5 megapixels (Nexus One) versus 18.5 megapixels (iPhone 3Gs). But here's the problem...the Adreno 200 inside the Nexus One had a peak fill-rate of 90 megapixels. The only thing hardware accelerated back then was the notification shade. Apple had significantly less overhead because it wasn't trying to do as much.
Lastly, there's a "communication" issue. Apple deals primarily with native apps, while Android deals primarily with Java (via Dalvik) and HTML 5 (through a Java interface...through Dalvik). Apple's approach is like two guys communicating with English. Android's is like two guys speaking different languages using a translator, at best. Their HTML approach is like four people communicating, each speaking two languages, trying to relay messages between person 1 and person 4.
Don't get me wrong. Android is a far superior operating system when it comes to what's under the hood and what can be done. But Apple has always prioritize UI interaction, to a fault. As a result, iOS has always been and likely will continue to be the more fluid operating system.
Summary: This is a relatively simplistic explanation. There are many more issues that come into play, but the overall theme is that Android does more, causing more overhead, and it often does more with less.
