The BenQ SW271 should be covering a much wider gamut than your results.
This review at ephotozine.com states the following:
The SW271 features a thin bezel and offers 99% AdobeRGB, 100% sRGB, 100% Rec.709, plus a High Dynamic Range (HDR) mode.
It's unclear if those are test results or specifications.
At Tom's Hardware the SW271 was actually tested to cover all of the above claimed color spaces.
The monitor is only one link in the color management chain, though. If the video card or the Spyder5PRO application is converting everything to fit in the sRGB color space before sending it to the monitor, the areas of Adobe RGB not also contained in sRGB space will not be sent to the monitor and the measurement using Spyder5PRO will reflect that.