time stamp and durations in postgres log file

in my Postgres log file, I see something like this:

usename dbname, .., logtimestamp1... <execute> ... insert ....

then the next line:

username, dbname, ... logtimestamp2... duration: 0.2 ms

My questions:

1. Is this 0.2 ms how much time the insert takes to finish?

2. does "logtimestamp2 - logtimestamp1 " have to be equal 0.2 ms?

I am trying to create a statistics report to tell how much all database transactions take all together out of a large log file. I added up all the duration times and this number is bigger than the time span (logtimestampFrist - logtimestampLast). It does not make sense to me, probably I did something while calculating? But I would like to sure that the line with duration time is really for the transaction in the line above it.

Thanks,
Jessica