employers with non-standard open enrollment period dates

I’m trying to figure out what the incentive is for an employer to have a non-standard enrollment period. My theory is that since it is legal for employers to choose their own open enrollment periods, having an open enrollment period in March may contribute to less of the employees participating.

I don’t fully understand this because the deductible calculation is from January to January. So if you join the plan mid year you would not have a full 12 months the first year you are on the plan which means that potentially you could occur more out-of-pocket expenses because the deductible will have not been met as soon. This really seems like a truly devious and legal way for an employer to make it more difficult to get affordable health coverage. The planet itself is pretty good and if you save up and meet your deductible (which happens to be the same as the out-of-pocket) in the first 3 months of the year then everything else is covered 100% for the entire year. However since they have the non-standard enrollment period it makes it more impossible during the first year.

Has anybody else had experiences with employer open enrollment periods that are entirely contrary to what everybody else is doing?

Thank you for any insight into this.