If the lnet_health_sensitivity was also used to increment the interface health, then there would be no point in having a variable lnet_health_sensitivity value. It would mean that there are N health decrements and the same N health increments for an interface. Also note (AFAIK, but Amir to confirm) that while the retry interval is 1s, it will increment the health for every successful RPC sent/received.
The reason that lnet_health_sensitivity=100 for decrements, but 1 for increments, this implies that the interface can only lose 1/100 = 1% of messages on that interface for it to continue to be in use. If it fails RPCs more than 1% of the time it will decrement faster than increment, which is good because you don't want to be using that interface. If it fails less than 1% it will generally remain in use. This "minimum acceptable failure ratio" is tunable by lnet_health_sensitivity.
Your docs are wrong 
It states at
https://wiki.whamcloud.com/display/LNet/LNet+Health+User+Documentation
that the health increments every time interval which is one seconds. By those docs it could be 1000 seconds before the interface is seen as healthy.
Amir Shehata (ashehata@whamcloud.com) uploaded a new patch: https://review.whamcloud.com/36920
Subject: LU-12303 lnet: recover health at same rate as dec
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 601be615b8409dc74f2f6e5c49fe0810bc443a73
Oleg Drokin (green@whamcloud.com) merged in patch https://review.whamcloud.com/36920/
Subject: LU-12303 lnet: recover health at same rate as dec
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 1d94a29dbc018fd00aa1c8a7a7ae343e0c9a4b83
Landed for 2.14