I have noticed the same thing with other cars owned in the past, but not with my 124.

One would think that the thermostat would be consistant reguardless of ambient temps, but other dynamics are at play, oil takes longer to warm, very cold air effecting etc. I doubt you have anything to worry about but when was the last time you replaced your thermostat?

This morning it was 2 below and have had to start my engine the last couple of weeks without the aid of the block heater as it no longer functions. Two glows and holding the starter a bit longer than usual, it fired off! BB