Why my subtitles are not counted ? 300 lines missing 
That could mean that your subs began after you did the 300. After you sub more, then they will be counting them!
On your profile counter it states 6300 subtitles, if you calculate them from your contribution it’s 2979, so missing are actually 21. Those could be of a channel, who was taken down, then the number will stay on the counter but the project will be gone from your profile overview.
You could say that your profile counter in comparison to the private counter is 15 subtitles short. If the system doesn’t adjust the number, in the next, let’s say 48 hours, you should write a request to Viki.