mIRC v6.3 was released in 2007, over five years ago, and there have been hundreds of changes and fixes since then, so it is difficult to say why that has changed. That said, I recall that there was a bug in older versions that caused some characters to be displayed in a font slightly larger than the rest of the text in some contexts, which was fixed in subsequent versions. Please note that you should not be using mIRC v6.3, since it has known remote exploits. You should at the very least be using mIRC v6.35.
