Sub-subscripts in strings cause different spacings than subscripts
The subscript is interpreted as Times[Subscript[H, 2], O], and Mathematica uses spaces to denote multiplication. You can work around this by using the ZeroWidthTimes option:
The subscript is interpreted as Times[Subscript[H, 2], O], and Mathematica uses spaces to denote multiplication. You can work around this by using the ZeroWidthTimes option: