Front and rear indicator bulbs are usually 21W. The flasher relay will be expecting 2x21W plus the 5W side repeater, and the higher electrical resistance resistance in 21W bulbs is how the flasher speed works.
I searched for images of a T10 bulb and saw pictures of what I would call a 501 bulb, normally used for sidelamps.
This is puzzling.
I've stumbled into TechS13 which is not my comfort zone.
Is your S13 using the standard lamp clusters and positions, or has something weird happened?
This is what I would call a standard indicator bulb: