HSA HDPD December

My son started his HSA account in April 2024. It is a HDHP.

 

We noticed that if December is checked and no other months are checked as "Self Only" HDHP coverage the MAX contribution is $4,150, but if April through November is checked another max contribution limit about half of the $4,150 is calculated.

 

I guess we are confused as to why if only December is checked the Max Contribution Limit ($4,150) is calculated?