What are the conditions for using skeletal replacement ('a') nomenclature?
Rule P-51.4, not P-15.4, specifies the conditions under which skeletal replacement nomenclature generates preferred IUPAC names.


  P-51.4.1.1 Skeletal replacement (‘a’) nomenclature rather than substitutive or multiplicative names must be used to generate preferred IUPAC names for acyclic structures when four or more heterounits are present in an unbranched chain containing at least one carbon atom and when none of the heteroatoms constitute all or part of the principal characteristic group of the compound.


The former compound has only three oxygen heteroatoms, not enough to require the use of replacement nomenclature.