We've done more investigations, and I think this is all about HTML formatting tags left over from copy-and-paste actions.
Even though you see plain text, and you might be using a plain "text editor", if you copy text from another system into the WYSIWIG editing window on G2G, you may be copying formatting tags that take up space.
For example, you might see this:
SQUIBB, William Head Widower M 40 1821 Coal Porter Weymouth, Dorset
But the character counter on G2G sees all this:
<div>SQUIBB, William<span class="Apple-tab-span" style="white-space: pre;"> </span>Head<span class="Apple-tab-span" style="white-space: pre;"> </span>Widower<span class="Apple-tab-span" style="white-space: pre;"> </span>M<span class="Apple-tab-span" style="white-space: pre;"> </span>40<span class="Apple-tab-span" style="white-space: pre;"> </span>1821<span class="Apple-tab-span" style="white-space: pre;"> </span> Coal Porter<span class="Apple-tab-span" style="white-space: pre;"> </span>Weymouth, Dorset</div>
Next time you experience the problem, try this: use the "Remove Format" option in the WYSIWYG text editor. It's the little eraser icon in the command bar. That should give you more space for text.