Interesting Band Gig Income for the Grateful Dead in 1979

I'm over 400 pages in my draft of my book on The Economic History of the Grateful Dead and I've discovered a fascinating fact.

While the Dead ended up playing 52 shows at Madison Square Garden in New York for the first two they were vastly underpaid.  

With about 20,000 tickets sold always at the Garden, and about $9.28 in average ticket price, the band got less than 20% of ticket sales.   Most gigs for the band were about 45%-50% of ticket sales in this era.  

This got fixed in September and remained about half for the next 50 shows.

I'm curious why the first Garden shows were so low paid.