Because there is a difference between categorical and a relative qualified classification in reasoning and language.
Ie take the range of colors considered to be 'dark'. They are all categorically classed as dark, but their relative differences qualify them to be classed as to 'lightness'.
Another example is of a sick person who may one day be experiencing fewer bad symptoms than expected and says that he on that account feels good. He is still sick but is less sick than otherwise and therefore in a relatively qualified sense he classifies his state as good because it qualifies to be called good on basis of being better than.
There is a definitive sense and a qualified sense of classification, this is evident in Sutta texts, ie here;
In This Very Life
“Reverend, they speak of ‘a teaching visible in this very life’. In what way did the Buddha speak of a teaching visible in this very life?”
“First, take a mendicant who, quite secluded from sensual pleasures … enters and remains in the first absorption. To this extent the Buddha spoke of the teaching visible in this very life in a qualified sense. …
Furthermore, take a mendicant who, going totally beyond the dimension of neither perception nor non-perception, enters and remains in the cessation of perception and feeling. And, having seen with wisdom, their defilements come to an end. To this extent the Buddha spoke of the teaching visible in this very life in a definitive sense.”
In the Dhamma all feeling is unpleasant in a definitive sense and feelings are also classed as pleasant, unpleasant and neither pleasant nor unpleasant in a qualified sense. This is evident in sutta such as;
"Monks, there are these three kinds of suffering. What three? Suffering caused by pain, suffering caused by the formations (Sankhaara-dukkhataa), suffering due to change. It is for the full comprehension, clear understanding, ending and abandonment of these three forms of suffering that the Noble Eightfold Path is to be cultivated..."
"Pleasant feeling is pleasant in remaining, & painful in changing, friend Visakha. Painful feeling is painful in remaining & pleasant in changing. Neither-pleasant-nor-painful feeling is pleasant in occurring together with knowledge, and painful in occurring without knowledge."
"If someone were to say: 'This is the highest pleasure that can be experienced,' I would not concede that. And why not? Because there is another kind of pleasure which surpasses that pleasure and is more sublime. And what is this pleasure?
Here, by completely surmounting the sphere of neither-perception-nor-non-perception, a monk enters upon and abides in the cessation of perception and feeling. This is the other kind of pleasure which surpasses that pleasure and is more sublime.
"It may happen, Ananda, that Wanderers of other sects will be saying this: 'The recluse Gotama speaks of the Cessation of Perception and Feeling and describes it as pleasure. What is this (pleasure) and how is this (a pleasure)?'
"Those who say so, should be told: 'The Blessed One describes as pleasure not only the feeling of pleasure. But a Tathagata describes as pleasure whenever and whereinsoever it is obtained.'"
"Excellent, monk. Excellent. These three feelings have been spoken of by me: a feeling of pleasure, a feeling of pain, & a feeling of neither pleasure nor pain. These are the three feelings spoken of by me. But I have also said: 'Whatever is felt comes under stress.' That I have stated simply in connection with the inconstancy of fabrications. That I have stated simply in connection with the nature of fabrications to end... in connection with the nature of fabrications to fall away... to fade away... to cease... in connection with the nature of fabrications to change.
In the Dhamma cessation of perception & feeling is pleasant and everything else is dukkha in a definitive sense but the dukkha can be classed as to pleasantness in a qualified relative sense.