Why does uniprot sometimes not label single-pass transmembrane proteins as "anchors"?
A membrane anchor is close to the end (N-terminus or C-terminus) of a protein. For "Single-pass type I membrane proteins" the transmembrane region is not necessarily close to N- or C-terminus, which is why single-pass type I membrane domain are not labeled as anchors. 

Instead, relevant entries usually have a signal peptide annotated.

Note that we have replaced the example for single-pass type I membrane protein by http://www.uniprot.org/uniprot/P14778#subcellular_location which is more relevant (the modified version of the documentation will be available with release 2016_01).