The general problem is, that any and every WP Theme behaves slightly differently. All cforms CSS themes have been tested against the default themes that come with WP.
cforms CSS is deliberately not overruling per-se all other CSS declaration, and as such may be victim to more general/global settings in your specific WP theme.
In other words, if your WP theme isn't (properly) designed you'll see the one or other effect on your forms' layout.
The fix however is easy and requires two slight adjustments of the width part (bump it up a couple of pixel), here:
label.cf-group-after { …
& here:
label.cf-group-after span { …