Abracon AB1805 RTC IC - additional resistance on VBAT pin?
That is a bit unusual - I suspect it is to control the charge current to the backup source if it is a supercap.
The data sheet doesn't seem to give any insight.
Many batteries, including CR2032 type cells, are limited in the capability for accepting current being injected. One of the other configurations where the power is applied to the RTC using VCC with a FET to isolate the RTC when no primary power is available would seem to be better when a lithium-cell is used as backup.