I honestly thought that this wasn't doing well at the box office at all.
Me too, George - I had an offer come through on email for it the other day. It was good enough for me to actually think about booking this time (usually the price drop is negligible for this show, so I ignore the offers!). So I assumed it must be suffering a bit in terms of sales.
Sometimes an extension is a last-ditch attempt though - hoping that enough people will want to book all the way into next summer to boost the coffers of the more immediate future. The length of this extension combined with a cursory read of the last few posts, I wouldn't assume that it's necessarily a good sign at all.