Hairdressing rant

What is it with hairdressers?! They often don’t understand that one centimetre off the length means one centimetre off the length - please!! They nearly always cut off much more. My hair is collar length and I had it cut today. I indicated how much I wanted trimming off with my thumb and first finger as the word “centimetre” or “half an inch” seems to mean nothing. The hairdresser even agreed with me that that was all that was required. So what do I see on the floor afterwards? About an inch and a half of hair. The photograph I showed the hairdresser - twice - for the style I wanted wasn’t adhered to without a word as to why not. Why do I often come away from the hairdresser without the result I want, even though I’ve tried my hardest to explain my requirements? This was a new hairdresser I tried today, but it has happened before with other stylists. Frustrating.