"R-value" is a measurement used by many mattress manufacturers to measure the warmth of their mattress.
Most people don't realize that EN tests (temperature ratings for sleeping bags) are all based on a mattress with an R-value of 5. If you pair your EN rated bag (of any design) with a mattress of a lower R-value (the vast majority of mattresses sold have lower R-value), your bag will NOT meet its EN rating for warmth.
In the last several years "air mattresses" have made a huge resurgence, and even self-inflating pads have gone on a foam-reducing weight-diet, negatively impacting their R-value. The unfortunate result is that many people cripple their 3 season bag with a "summer" mattress of R value of 1-3. Its no wonder you see so many 15 degree "winter" bags used for 3 season use.
The industry does not care, and they will sell you any mis-matched system you are willing to buy. But Zenbivy will not offer mis-matched mattresses, or build a winter bag and market it as a 3 season bag to make up for insufficient mattress.
Here's the fact: An R-value of around 5 is a "3 season" mattress, and is what is used to generate EN ratings for bags from about 5ºF to 50ºF. The majority of mattresses that are sold are summer-only, at R values of 3 or less.
The mattress and sleeping bag are a system, and unless the R-value of your mattress is 5 or above, it will not match your EN rated sleeping bag.
So if you have a 15 degree EN bag and are still cold in 3 season conditions: check your mattress. The reason you are suffering is likely because the mattress cannot support the rating of the bag.