From the text manual entry:
b]Text rendering
Different browsers render text in different ways, and Construct 2's layout view also renders text differently to other browsers. This means you should expect the appearance of the Text object to vary slightly across browsers.
You should test your game in a range of browsers to ensure text objects display how you intend for all users. For more information see Best practices.
Unfortunately it's just a side effect of having lots of different text renderers in different browsers. There are some vertical align tools coming for the text object in the next release - that might help.