Why does my DS1923 fill up its datalog with 2048 data points when the data sheet says it can take 4096?

The number of data points stored depends on the mission settings for the DS1923. The part can handle a total of 8192 8-bit readings or 4096 16-bit readings. When measuring both temperature and humidity, instead of only a single temp or humidity reading, 2 readings are being logged simultaneously.  So, twice as much data is being stored.

Thus, for 16-bit high resolution settings while simultaneously logging both temperature and humidity readings, you can only take 2048 points before rollover. You must use the low resolution setting to be able to take 4096 points before rollover.