You’re computer calculates the whole dive not just the bottom time
Well, I don't know what tables/computers that you dive but I learned diving a table that said 60 minutes was NDL for air at 60 feet. My current computer says 55 minutes.
Modern computers calculate in real time. TTS, (time to surface,) is an estimated figure, based on current (real time) data about the dive, (and diving history) an assumed ascent rate, (potentially an assumption on what decompression gas will be breathed during the ascent).
If you deviate from the assumed elements used to calculate the ascent, , the computer will recalculate the TTS.
NOTE - this is why, when you have a decompression stop to do, and you choose to decompress deeper than the ceiling stated by the computer, e.g. decompress at 6m rather than the 3m, your decompression time will be greater than stated. Modifying your TTS. In the UK, we often choose to decompress at 6m, rather than the computers preferred decompression depth of 3m. Surface conditions often preclude shallow stops. Most UK divers allow for this in their dive planning. If I am on a no-stop dive, I will do safety stops at 6, 4.5 and 3m, the computer will not acknowledge the safety stop at 6m, but will once at 4.5m or shallower.
It should also be remembered that there are differing theories on how to calculate decompression requirements resulting in different no stop time for the same dive, dependent on table (theory used). This variation is increased in the dive computer market, where additional elements may be added, conservatism, padding, adjustment for number of repetitive dives, poor dive practice, etc.
A manufacture may have multiple models across its range e.g. Suunto, which uses RGBM, RGBM 2, Fused RGBM and Tech RGBM. Some of the Tech computers allow you to change models dependent on your preference, e.g. Bulhmann or VPM.
I quoted NST figures for a dive using differing mixes earlier in the thread, I qualified the statement by stating WHICH model was being used. Differing models, differing times!