Missing signal record?

I run the query:
TABLE = from signal in C2SIGNALS
where signal.SystemId == 30415311
&& signal.Symbol == "QID"
select signal;

I then export the returned data and sum up the BTO quantities. I get 18,495.

I sum up the STC quantities. I get 18,509.

We closed 14 more QID than we opened.

[BTW I tried to get data out of Explorer b/c the data of this system presented on the UI is hopelessly garbled. I sent a report about this to the help desk. Looks like the database is not accurate either.]