1997 Chevy Monte Carlo Compression test

I replaced the timing belt on this 3.4 L DOHC Vin X engine - then I did a compression test - since the old timing belt had broke - and I got the following reading- #1 = 170, #2 = 40, #3 = 0, #4 = 180, #5 = 30 and #6 = 65. Based on these readings I removed the heads and took them to engine repair shop. They did tests and said the valves/heads are good. QUESTION: What could cause these compression readings? Thanks