At least for me, vocal warm-ups such as singing ascending scales or patterns temporarily increase my vocal range upward. I believe those warm-ups would also work for you (and choral teachers keep making students use them in-class for this reason), so yes, I'd chalk these variations up to wrong technique to a certain extent.
Many factors can affect this. Low-grade inflammation is among them. If you suffer from respiratory allergies, even mild enough that you notice no other symptoms, you may find that the higher notes become difficult or unavailable.
Even lack of sleep can affect your range.
One thing to be careful of is gastric acid reflux. Singers may be especially ...