NeillM has just filed a pull request to fix this by improving the SQL query used to obtain the data - I've checked this for security but haven't tested it or looked at the performance implications - If others here could test this and let me know that would be appreciated - please also provide information on the Database type you have tested it on (mysql/postgres/MS SQl) etc
you can get the latest code with the patch here:
https://github.com/danmarsden/moodle-mod_attendance/archive/master.zip
This code hasn't been pushed to the version in the Moodle plugins db yet - I'll wait for confirmation from others before I do this.
thanks to Neill!