It's due to changes in air density and relative humidity which results in dew point temperature being different on different days. This changes the altitudes at which the effects of contrails will be noticed. Cloud or ice crystals can spontaneously form with the drop in pressure that occurs at high lift producing points of the wing. Jet exhaust also produces trails by the soot acting as a condensation point for the water vapor.

Every day is different, as evidenced by the synoptic chart with the isobars connecting points of equal pressure changing by the hour.

https://www.encyclopedia.com/earth-and-environment/atmosphere-and-weather/weather-and-climate-terms-and-concepts/dew-point