Great question — answering as the UphillTech team:
Yes this is normal, especially on bikes that have a short seat tube (all modern mountain bikes). Does this 26cm post require 26cm of insertion to be safe to ride or is the lower part just 26cm long? If latter, it'll very likely work even if it's raised 1cm. Drilling out the bottle cage boss is also a possibility to have more options in the dropper post choice. On aluminium frames it's a rivnut anyway so as long as you don't damage the hole you can replace it if you later decide to switch to a solid post. You can mount the bottle cage with a hose clamp if you want a second one.
Full answer by Torben: https://bicycles.stackexchange.com/a/100634 (CC BY-SA 4.0).
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