For any poet reputed to use ordinary, or in any way less elaborate or more colloquial language, readers will misjudge the poet's language as more colloquial than it actually is, adjusting their perception to the prevailing view.
The reasons are these:
Readers will read as they are taught. So if they are taught that Machado rejects ornate poetic language, they will not notice when he happens to use this language, even if he does it a lot.
Readers will read contrastively. So if there is a more ornate tradition, they will correctly notice that Wordsworth or Bécquer is turning his back on that, without noticing what the poet might have retained.
Readers are ideological. They think that less ornate language is better, so they are invested in thinking that Gil de Biedma or Cernuda are colloquial.
They will generalize from a few key instances of colloquialism, thinking that the entire poetic work is in conformity with the few textbook examples.
There is no such thing as ordinary language anyway. There is only language in various registers. Thus when reading a poem the conversational norm is rather diffuse in their perception. To see the poem's language as conversational they just have to not think very hard about what actual conversations are like.
Readers' perceptions of language are not very fine-tuned. Even trained critics and other poets who should know better make these misjudgments.