Aloft
Why do default Blizzard nameplates sometimes appear before Aloft updates them?
Let us call this phenomenon "style switching". There are several issues operating here, simultaneously.

The first thing to know is that the WoW client "recycles" nameplates. When they go out of range, or the unit dies, and the nameplate UI objects are not needed any more, the WoW Client repossesses them, and Aloft turns loose of them, and the WoW Client places them in a pool for re-use, so that they can be assigned to the next unit to appear.

Aloft remembers every nameplate it has ever seen, as a raw UI object, including the ones that have been repossessed by the WoW Client (that are sitting in the "recycle pool"), and it does attempt to initialize nameplates proactively, before they become visible (i.e. even when they are sitting in the "recycle pool". This is done at a user-configurable interval.

But the WoW Client does not usually re-assign a "recycled" nameplate to a new unit until the moment that unit comes into nameplate range. The usual data that Aloft uses to configure color, class type, and so forth are not available until the nameplate actually becomes visible to the user. So usually, even this proactive initialization does not do any good.

Another problem is that when zoning into a crowded area, or approaching a large number of units at maximum range, a large number of nameplates will come into visible range all at once. Aloft doesn't necessarily get a chance to complete any sort of initialization (proactive or otherwise) before the nameplates become visible. Given that Aloft incorporates a lot more functionality than just a health bar/text reskinning, this can "take a moment" (especially if you don't have a quad-core "extreme" processor and/or less than 4Gb of memory). What is more, there can be quite a bit of lag under these circumstances anyway (which impacts the initialization process even more).

Hence, there are many circumstances under which nameplates can be visible for a brief interval before Aloft can configure them (whether from their Blizzard defaults, or simply to set the color/alpha correctly, etc).

However, the customization that Aloft applies to them should remain, through this recycling process, so when they are assigned to a new unit and re-appear, much of the basic appearance should remain roughly the same as when Aloft last had control over them. Aloft cannot really make any assumptions about whether a nameplate has already been "customized", but their basic appearance when they first appear, in that brief interval between appearance and when Aloft resumes control, they should display as "Aloft-style" (except for things like color). Assuming Aloft has access to enough nameplates, and they have recycled, the WoW client should have a pool of them available (already customized by Aloft), and the frequency of this "style switching" behavior should be significantly reduced.