Why the difference in # of days remaining?

I have always purchased BDTS editions. Why is there a difference between the the various modules as shown in the attached file?

I had 307 days left on my Windows PC's but kept getting warnings about expiring products (probably because the other modules were "expiring".

I ended up buying a 1 yr extension and now my Windows PC has 604 days remaining!!  I could have avoided spending that $ now, as I use BDTS on Windows PC's only.

Whats going on with my licenses?