I wonder if it has to do with "t%delta" being much faster to calculate with delta=8 than with delta = 7, 6, or 5. If you miss the millisecond where t%delta == 0 then that send will not occur.
115200/10 = 11520 bytes per second / 27 = 426.666 packets per second or 2.34 milliseconds per packet
Try delta=4. That is also easy to calculate and it that value works it would tend to show that the unsigned long modulo is taking too much time and you are missing 2.3% of your millisecond intervals