reekta

Spoilage speed depends on the overall temp of the season so it's normal that spoilage rate changes throught the year.
I've seen some people living higher altitudes make rooms at top of the mountain (where is really cold generally) to reduce the spoilage rate, i don't think that counts as cellar but as long as it's covered with solid blocks it counts as a room which also reduces spoilage which combined with the low temp of the mountain they should last pretty much same as a normal cellar underground i suppose, should test it sometime.
You can also cover all 6 sides of the storage vessel to reduce spoilage rate but honestly it's not really worth it, the difference of x0.05~ is really not that big of a deal especially if you are playing singleplayer. In my hours of the game i've always had so much food from farming that regardless of how much i try to reduce the spoilage some of the food will rot anyways and it's not that big deal since you can harvest your farm around 2 3 times(for some crops like turnips) before winter even with a pretty basic farm setup.
After some point i even just stopped using storage vessels for veggies and grains because i got so much food lol, instead using crates which has same spoilage rate as any other storage but is way more efficient and compact(and they look cooler //media.invisioncic.com/r268468/emoticons/smile.png ).
If you really want to minimize the spoilage you should also not use any wood type block in the cellar, they increase the overall spoilage by a bit, but like i said it's not that big of a difference honestly.