Speaking from a purely objective standpoint, we as humans are all differently abled from one another. Some people are better at math than other people. Some people are better at public speaking than other people. Some people are better at cooking or even remembering to cook than other people. Some people walk and some do not, and of those people who do walk, not everyone walks in the same way. When referring to groups of people, there is nothing inaccurate with saying that within the group, each person is differently abled. This is true regardless of how many disabled people are in the group or if there are no disabled people in the group.
The problem arises when the term "differently abled" is used to refer to an individual disabled person.
Firstly, calling someone "differently abled" is euphemistic. It is borderline cutesy and it diminishes the actual experiences of disabled people. It suggests that the term disability should be uncomfortable and therefore should be avoided. What this does is further increase stigma against disabled people by discouraging discussion about disability and what it means to be disabled.
Secondly, using the term "differently abled" to refer to disabled people actually reinforces the idea that there is one normal way to be human -- that there is one normal way to move, one normal way to communicate, one normal way to sense, one normal way to feel, one normal way to learn, and one normal way to think. It does not perform its intended purpose of suggesting that all people are different and that this is okay. It suggests that only disabled people, who must now be called "differently abled" instead, are deviant or defective from this normal human model, and it suggests that there is in fact a correct or right way to be "able." It supports the false idea of the normal body/mind, which is what "differently abled" is supposed to undermine, and thus it fails in its supposed purpose.
Thirdly, the phrase "differently abled" ignores the reality that disability is the result of a complicated interaction between individual people's bodies/minds and social, cultural, and political structures that actively work to disable people with atypical bodies/minds. This happens because "differently abled" suggests that disability is one person's individual problem while also denying the impact of systems that privilege people with typical bodies/minds while marginalizing people with atypical bodies/minds.
When I say that I am "disabled," I am not putting myself down, insulting myself, suggesting that something is wrong with me, or making a negative statement about myself. I am staking a claim in an identity that is important to who I am as a person. I am recognizing that my mind/body function atypically, and that because of this, I am constantly forced by mainstream social/cultural attitudes and the laws and policies that enforce them to choose between being othered (and then discriminated against or outright harmed) or accepting the idea that I must hide who I am by passing as an abled person.
By calling myself disabled, I am rejecting the idea that it is wrong to have a mind/body like mine.
When I say that I am "disabled," I am not reducing myself to my disability, just as I am not reducing myself to my gender or my race when I say that I am a woman or that I am Asian. Being disabled is one part, albeit an important part, of my multifaceted identity. Each of these parts overlaps with each other, blurs into each other, and intersects with each other; they are not separated or disparate. It is important for me to define who I am, and being disabled is an important part of how I define myself.
I do not believe in referring to disabled people as "differently abled" because this language only serves to reinforce oppression of disabled people by systems that marginalize atypical bodies/minds.
Photo: I (Lydia) am standing in front of several people walking down a stone path, holding a large white poster with the colored block letters "Autistic & Proud" in multi-colored rainbow handwritten marker. I'm a young east asian person with short hair wearing glasses.