Most people don't judge by how much it's raining though, because that's rarely relevant to decision making.

People judge by the number of rainy days, because when it is raining (even if only a little) there are many activities that become limited, or more difficult, so if it rains all month you can't do those activities all month where as if it only rained one day but rained more water on that day, you'd just stay dry inside and go out the next day instead.

And Seattle averages 152 rainy days a year. By comparison New York averages 119 days, nearly a month less. New Orleans a mere 56. Boston 130. Mobile Alabama, 145. Pensacola Florida, 141. West Palm Beach Florida, 127. Lafayette Louisiana, 106.

So while it may not rain more in terms of absolute volume, it rains more in terms of frequency, which is the metric that actually matters in terms of it's impact on peoples lives (barring flooding). Hence why it is perceived as a rainy place, something everyone ignores so they can have their 'gotcha' moment when they say it isn't actually. Many of the cities often listed as 'rainier' don't have even a third of rainy days as Seattle has.