Are you at an age when presbyopia might be an issue? (Over 40?)
If so, you might experiment with some weak plus glasses for in front of the computer and see how that changes the measurements. Maybe use +0.25 or +0.5 if they let you get far enough away from the screen to keep you at a usable distance.
I’m guessing that the lens in your left eye might be stiffening with age and you’re probably using one eye more than the other when you look at the screen up close. Both lenses are probably stiffening but you’re probably measuring myopia on the snellen chart and seeing some blur from presbyopia on the near target. [???]
It could also be some kind of subtle convergence issue at near that you could have at any age (I seem to have some that I’m working on fixing.) You might try moving the screen around, hold it to the bottom right when you look through the left eye at it (that should pull up some accommodation reserves because you have to use more convergence when your left eye is looking down and right) or try looking at the screen up and to the left through your left eye (to get it to stop accommodating).
Depending on which way your eyes are pointing they will want to accommodate more or less. When you’re looking at the snellen chart far away they’re probably not accommodating.
Your left eye is better at distance so it’s probably having to accommodate whenever you use the phone or computer (regardless of how old you are). It might be accommodating too much when you’re trying to measure your cms.
Sam’s got the phone out, time to accommodate!