Lsof: no pwd entry for UID

I know of fuser, see if is available on your system.

Without seeing the actual output it's hard to say exactly what is happening but I'm guessing it's caused by the fact that the lsof -p PID command is just printing out files that are opened by the specified PID while the lsof | grep PID is printing out any lines where 'PID' is located anywhere in the line. For example, if you're searching for PID=123 then your grep option will also print lines for PID's 1231, 1232, 1233, etc. as well as any files which are located in folders with 123 anywhere in their full path.

EDIT: In your specific example the difference is that lsof is ignoring files opened by a thread. If you look at the output in the grep example the third column is the 'TID' or Thread ID. Lines without a TID match what you were seeing when you used the -p option. Lines with a TID (ie. lines opened by other threads) are the extras.