And how is that deceiving? They're not implying anything (your use of the world necessararily). Aren't you deceiving yourself by making yourself beleive it's in-game? Or deceiving yourself by making yourself beleive that the developers say they're showing you in-game footage, while it's in-engine (basically making youself beleive that developers are lying to you on purpose) ?
When dev's show in-engine footage, it's to demonstrate what their engine is capable of. From that point on, it's scaling to meet the hardware performance. To generalise it, the in-engine is comparable to a PC game at max setting and the in-game footage is that same PC game, on inferior hardware on a lower setting. Exactly the same tech, but lower quality.
As for Halo 3's reveal trailer, with the shiny MC, the spartan models in the Halo 3 beta did look like a lower quality version of that model. What we see in the final game is a choince of artstyle. As for the reflections in the visor go, didn't happen. Cost too much resources, although the visor does reflect images selectively in the singplayer campaign.