Because a higher pressure creates harmonics that annoy the heck out of you...at least that's what the Vintage Air tech told me. He said to keep the pressure as low as possible while still staying in the range.
I first had mine charged by the local AC specialist and he had it charged to the high number. My system shuddered at max cooling settings, but it stopped when he lowered it to the low number.
You may have different results.
