There's your reason, fuel injected prior to opening will vaporise earlier but it leaves a trace of carbon dust.
It also gets blown back up the runner at low engine speeds during the valve overlap period.
The small amount of
oil from your pcv is the other contributing factor.
Injection should really occur after the exhaust valve shuts, say 40 ATDC or later at low engine speed, then bring them on earlier as the engine speeds up.