Consistency of light?

One thing that nagged me as I watched was how either Tim or Vermeer could keep the light from outdoors consistent throughout the process? The film points out that the outstanding qualities in Vermeer's work are the shades and gradations that humans would otherwise not register, so I would think any subtle shift in the exterior light would throw the whole thing off a bit, on a much greater scale than the out-of-place chair that happens partway through.

Tim's studio faced north as Vermeer's did, but is this really all it takes to neutralize the sun's gradual movement throughout such a lengthy process? Not just the angle and color of the light in the course of a day but the quality of the light throughout the months-long process. Holland has quite a bit of rain, and even San Antonio has the occasional cloud passing overhead, so how did either of these two men, assuming Tim's hypothesis is correct, work around this problem? Perhaps it's just patience and only working for the same few hours each day and only if the sky is clear, but it seems like this would be a large obstacle to me.

Or perhaps it works for the very reason that humans wouldn't notice the more subtle changes anyways, much as Tim's accidental nudging of the lens partway through isn't noticeable either.