Why do my periods vary so much each month?

I am wondering why it is that sometimes I am up to 5 days late and others it is on time precisely.I also have minimal pain some months and others,I am bedridden for a day or two with constant gripes. I am 31, healthy, no evidence of hormonal imbalances in last blood test, no endometriosis in my last laparoscopy. No poly cystic ovaries etc, so why the huge variation? I do not drink, smoke either.