In ERA5, "2 metre temperature" is an instantaneous parameter and is available hourly, from both the analyses and the forecasts. The parameters "Maximum temperature at 2 metres since previous post-processing" and "Minimum temperature at 2 metres since previous post-processing" are the maximum and minimum in the last hour, and are available from the forecasts only.
The data on the CDS disks, contains analysed parameters, if available, otherwise it contains forecast parameters. Therefore, on the CDS disks, the "2 metre temperature" is analysed, whereas the "Maximum temperature at 2 metres since previous post-processing" and "Minimum temperature at 2 metres since previous post-processing" are forecast.
The forecast model in the version of the ECMWF Integrated Forecasting System (IFS) used in ERA5, CY41R2, has a cold bias in most regions over the globe, w.hich is at least partially corrected by the analysis system. This forecast cold bias means that the analysed "2 metre temperature" can be larger than the "Maximum temperature at 2 metres since previous post-processing".
Given this inconsistency in these three parameters on the CDS, we recommend in general that the hourly "2 metre temperature" be used to construct the minimum and maximum over longer periods, such as a day.