They're only subjective if you haven't got enough reliable measurements. Since the manufacturers mislead on much of the data, it takes good equipment to provide an objective measurement.
I'm not an graphics or monitor expert or engineer, but intuitively it does seem there is some subjectivity with monitors.
Let's say you had quality equipment and measured three current high end monitors of a given size from different manufactures (say, HP ZR line, Dell Ultrasharp line, and Nec Professional line).
After getting a set of measurements for each model of say pixel pitch, brightness rating/luminance, contrast ratio, viewing angle, response time, color gamut, display colors, refresh rates, color accuracy, etc... it's likely that one monitor would come out "on top" depending on what evaluation criteria one was using ie; a monitor review roundup, though it's also unlikely one model would dominate in every numeric category.
As a reader of a review and user looking to purchase one of those models, a person may read through the review and purchase the "top" rated model, assuming it had the features/input ports (s)he was looking for. Though there is a chance if that same user were to read the review, go to a store which happened to have 2 of 3 of the models reviewed, and looked at the models side-by-side (and we all know how retail stores calibrate monitors on the showroom floor), (s)he may favor/purchase the "2nd" or "3rd" place model because they thought the image quality simply looked better.
When I was looking at HDTV's a few years ago, I read a lot of online reviews, and for me it essentially came down to a Samsung or Sony in the price/feature range I was looking in. Though I ended up purchasing the thing online due to price, I did go to a few local stores and actually looked at both models side by side, and to my eyes, the Samsung looked better. Some online reviewers favored the Sony, some the Samsung, but in the end I picked the one that looked better to me.