As the title says the corruption resist is being accounted for the healer essences' corruption resist, tested it on multiple toons with healer spec, and with both the Spirit of Preservation and Unwavering Ward. Both have this issue, it says +87, however the cloak is only +77.

The same characters on damage spec or in healer spec WITHOUT the essences that give +10 resist working perfectly fine and showing +77 in character tooltip.