Preside embeds a self-hosted version of Open Sans for use in the admin.
However, the two variants embedded are Open Sans Light (300) and Open Sans Regular (400). This means for any weights heavier than this (e.g. bold) it is left up to the browser to extrapolate from the available fonts, and the results are variable.
In Chrome on the Mac, for example, the difference is clear. On the left is Open Sans text in Preside; on the right is the same list of fonts on Google Fonts, but these use the correct variant font files:
I would recommend adding an extra weight - Open Sans Bold - at a weight of 700. This gives a much better appearance, and actually makes bold text appear bold in Chrome on the Mac!
I would also recommend adding italic variants for each of these three weights, as italic text looks much better with the genuine font than with the browser just fudging it by distorting the normal variant.
Each variant weighs in at around 22K, and they will be cached by the browser after first load, so there is very little extra overhead involved in adding them in; and the Preside admin will look much more attractive!