Number of songs on iPhone doesn't match what I see in iTunes

When I open iTunes and plug my iPhone into my PC, the number of songs iTunes tells me is on my iPhone is slightly different than what the iPhone itself tells me. This is true for the total number of songs and the number of songs in each of my playlists. The iPhone says the total number of songs is greater, but iTunes tells me that my playlists are greater than what the iPhone says for each one

• Total:
  • iPhone: 15,077
  • iTunes: 15,067
  • Variance: 10

• Playlist 1 (brand new playlist):
  • iPhone: 5,595
  • iTunes: 5,596
  • Variance: 1

• Playlist 2 (older playlist):
  • iPhone: 314
  • iTunes: 313
  • Variance: 1

I do not have my iPhone synced with iTunes, as I have a lot of music in iTunes that I don't listen to but want to keep for one reason or another. I also do not currently have Apple Music (I had a 6 month trial a couple of years ago but didn't keep it). My music library is a combination of CD rips and iTunes purchases. I manage the music on my iPhone by plugging it into the computer. I drag songs from iTunes to the iPhone, delete songs from the iPhone, and add to/delete songs from playlists that way. Occasionally I find songs that didn't make it onto my iPhone when I dragged them over from iTunes.

What could be causing these variances? And, is there an easy way to find out which songs are/are not where they should be without going through 15,000 songs line-by-line?

Thanks!