I just decrypted an exotic boots engram I picked up 10 minutes ago.
I am light level 392 (right on the brink of 393). My existing boots were also 392.
The engram decrypted at 391
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This is (somewhat) normal. Exotics can decrypt anywhere from -1 to up to +3/4 of overall light level. So getting them at 391 when you are 392 isnt uncanny.