Laygridder overwrites the default editor styles in all TinyMCE instances, also the ones outside laygridder. I think laygridder functionality / styling should be limited to the laygridder editor only.
TinyMCE editor style overwriting default styles
This is because the textstyles feature is being used and the "Default" textstyle is used by default for the tinymce texts.
You're right, this should be optional. I'm writing it down!
Hey with the new version 1.1.0 this is now fixed!