Either it's a marketing mistake or a bug, but the iPhone 5s' new slo-mo feature apparently isn't recording at 720p as advertised. When compared with output from a Samsung Galaxy Note 3, it seems to be taken at 480p instead, then upsized to 720p.
One of the new features of the iPhone 5s camera is the so-called slo-mo video recording, which lets users record a video at 120 frames per second which can then be viewed in slow motion to get the intended effect. According to the website, the video recorded is 720p quality, but some users have noticed lower quality output, leading some to investigate the matter using the Samsung Galaxy Note 3.
It might be be comparing apples to oranges, considering the Galaxy Note 3 does not have a similar slo-mo feature, but the evidence is still worth examining. A video taken using the iPhone 5s slo-mo mode in 720p ended up lacking some details when compared to a similar shot using the Galaxy Note 3, also recorded at 720p. On the other hand, a video recorded with the Galaxy Note 3 at 480p then upscaled to 720p gave an output quite similar to that of the iPhone 5s. The testers noted that the iPhone can't record non-slo-mo regular videos at 720p, so those can't be compared.
We'll give the benefit of the doubt that this is merely a software bug that can be fixed with an update soon. Either that or Apple hasn't been clear on the feature's intended behavior. Amusingly, the slo-mo effect does indeed record videos as 120 fps, resulting in videos that practically cannot be viewed on PCs or any other player, unless forced to run four times slower than normal.