Are you saying that because you believe that since you don’t listen to audio on your phone that the 12-14 hours must be screen on time? If so, that is completely false. Usage time in iOS is essentially any time the processor is actually being hit with background processes or tasks to perform (phone calls, messaging, background refresh, etc).
The only actual way to measure screen usage is to add up the on screen time for each app in the battery section of settings. However, this poses its own issues as you only have stats for the last 24 hours or 1 week. Therefore you’d have to have a standby time of exactly 24 hours, then add up the screen on times to get your accurate measurement. For extrapolation purposes you can take the total number of minutes and divide that by 1-%remaining(decimal format). For example, my screen on time for apps adds up to 212 minutes (although 114 of those are on the phone which for some reason counts) and I have 53% left. That extrapolated out would mean I get a solid 7.5 hours of screen on time with my X (212/(1-.53)). My current usage stats though show usage of 341 minutes, which I could extrapolate out to getting 12.1 hours (341/1-.53).
All this is academic though since there is no way to predict how I will use my phone for the remaining 53% of battery life. Also, when counting screen on time I’m really only at 17 hours standby instead of 24, but I need to have my phone charged for the morning so I’m plugging it in; effectively clearing that day out. I could see the 8+ getting slightly better battery life than the X, but certainly not a >2 hour difference with usage patterns being the same; unless you had a bugged install of iOS.