I am not sure if this is a bug or not. When I use something like sphere.position.y = string1; if string1 can be converted into a number it will , and this value is used to update what you see on the screen. So far so good.
Now when I did sphere.position.y += 1; I expected it to use the internal number it used to position ittself correctly on the y axis and to add 1 to it, but instead used string1 as a basis, and the result is far off.
Why does sphere.position.y = ... not transform into a number both sides the internal number but also the public number; what you get back when you look for sphere.position.y again.
See in the console, in this playground
http://www.babylonjs-playground.com/#1QFFPV#1