GnuCash exports transactions created or modified after the date set into SINCE, which is not necessarily the same date as the one shown when the transaction is listed or edited. For example, lets say it's 2016-11-20 and you create a new transaction setting its date to 2016-11-18. Now, if you do an export since 2016-11-19, you'll get that transaction exported. The reason is that you created it on the 20th, which is the date that is really checked.
This is done in this way because it's meant to be used to sync changes with GnuCash desktop.
Take also into account that when you import a file into GnuCash Android, all its transactions get the current date as creation date. So, any export with a SINCE date previous to the date you imported the file will include those transactions.