Can not confirm an issue here.
This has something to do with the round up from decimal places.
Not every armor piece has exactly 1 resistance internally but shows 1 (rounded).
For example:
Yoggite has internally a Heat res from 0.6 so each piece shows 1 (rounded)
wearing 2 pieces means 1.2 but still show you 1 in total (rounded)
wearing 3 pieces means 1.8 but shows you 2 in total (rounded)
wearing 4 pieces means 2.4 but still shows you 2 in total (rounded)
wearing all 5 pieces means 3.0 and shows you 3 in total.
Greetings
Sir Bowen