That's why it's a rule of thumb. It doesn't always turn out to be exact, or even near the actual numbers. The reason that it exists is because it's the target for most producers. A producer chooses the theatre (with corresponding rent costs) and prices seats in order to make a profit - obviously I'm only talking about commercial productions. Seats are typically priced in such a way that if a theatre earns around 50% of full priced revenues, the show will break even. Sometimes that number is 40%, sometimes it's 60%, but 50% is the generally accepted approximate average. Why you might ask? A number in that neighborhood means a show will be making money as long as it's a moderate success and producers can discount as long as sales are good and still make a profit. If you require a much higher gross potential percentage to earn a profit, discounting will kill the show and you need to be near capacity every performance, which isn't realistic for most productions. If your required gross percentage is much lower, it likely means artificially-high ticket prices are inflating the overall potential gross and more tickets and ultimately greater revenue will be found by lowering prices.
It's often the case that shows in huge houses (like Follies) are planned by producers to break even at far below 50% since they don't anticipate sellout crowds. On the Town coming into the Lyric will probably follow a similar model (I don't know the financials on it, so I'm just guessing).
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.