There is a limited amount of real estate at the bottom of a post since it's width is fixed. The site might be responsive to resizing horizontally, but the "information width" does not change. To this end, the flair box width is also fixed. A similar problem is experienced when you have a large number of badges (see the status-declined Display bug causes number of badges to be shown incorrectly).
Understandably Stack Exchange enforces some limit in terms of what can be displayed, but allows users the freedom to enter whatever they want for a name in their user/network profile. As such, some names are visually truncated, while others are not. Here's another example on Meta.SE: