Bitter Cucumbers

I was watching a cooking show and the chef made the comment that if you use a large basic cucumber (not hot house, persian, kirbys, english, etc.) before you peel it, cut off about half an inch of the ends and rub them on the opposite sides, then throw the cut off ends away and peel as usual. I normally buy persian, hot house, english or other types but I purchased some of these larger cucumbers. I did what the chef said and they were great. I did this a few times, and I take my time picking out long, slender, firm, green fresh cucumbers. I started buying these when I thought the price was right but then I thought taste them first to see if they were good to start off with. They were. I pickled about 4 gallons of persian cucumbers last week and tasted the ones I thought might be bad, they were and tried the trick. Did not work. All cucumbers were USA products and grown in California. My friend brought me some big cucumbers last night and we tried the trick. Well, she eats anything but it did not work. Has anyone heard of this trick or knows why it works sometimes? Maybe the age, the girth of the cucumber, not straight or even in size, any ideas?