The bulb takes a good fraction of a second to heat up and cool down. While doing so, the light is dimmer and redder, therefore exposing the paper less-effectively. So what matters is not just the total exposure time, but how many times the bulb was switched on and off to make the exposure.

If you have a mechanical shutter on your enlarger or it has a solid-state (LED) light source, then this effect should not occur because there is no warmup/cooldown effect, the light just goes on/off.