Certain molecules, like citruses or most fresh notes in general, evaporate very fast, so they only appear in the top. (Witness the whole debate and misunderstandings about the eau de cologne recipe).
Persistent notes are present at all stages, however, in the top, they are usually overwhelmed by the evaporating top notes.
But here are many perfumes that smell the same from beginning to end (such as most orientals). Patchouli, sandalwood, spices are clear and loud from the beginning in them.