It seems like your perspective boils down to truncation of small increments on DOT ticks.

Assumptions:

1. Damage is actually (not just visually) truncated for each ability separately.
2. You only invest in DTR in small increments.
3. Damage is truncated for individual ticks without crossover for "remainder" damage.
4. Damage without the delta DTR is an integer.

The first assumption is 50/50. By "separately", I mean with regards to all DOTs since they are stored as a single piece of data in memory. If Bio is getting +0.4 per tick, and Miasma is getting +0.3 or whatever, and Bio II gets +0.4, then the total would be +1.1. For your scenario of "DTR gets truncated out a lot", each separate spell tick would have to be separately truncated.

Also note that the "visually" part is important. There are already other data values in the game that have been demonstrated to be truncated for the UI but not in reality (specifically spell speed re: the GCD).

In addition, you are assuming your base damage per tick is an integer (re: #4 above). E.g. 40.0000 damage. This is highly unlikely since your WD is like 6x and Int is at >400. Whatever the actual damage formula is, calculating that down to "40" damage per tick is not going to result in a flat integer. It's just as likely that your 40.9 (truncated to 40) dot tick is boosted to a truncated 41 by 10 DTR (again assuming truncation is real and it's truncated per dot per tick).

The second assumption is a significant assumption, since no one makes gear strategy choices based on +/- 10 points, except for ACC since it has very specific thresholds. Generally if you're going to change your gear strategy based on stat valuation, they're going to swing in the 50+ range depending on gear availability.

The third assumption is minor. The fourth is pretty major and already discussed above.