40-50 dBa can be audible to a point of distraction, but is certainly not as loud as a vacuum cleaner. Your point about the chart being misleading is valid, as "Public rooms in hotels" is a very vague category when defining sound levels.
However, I disagree on 35dBa being as loud as someone talking or normal conversation. Remember that noise levels fall exponentially over distance and AFAIK there is no standard noise measurement distance for case fans, so noise readings can be very misleading.
For example,
this Apevia fan is rated at 24.7 dBa, but I found it considerably louder than my Antec 900 fans set on medium which produces 27dBa of noise @ 1 meter. After that, I've always taken sound levels of fans with a grain of salt.