It is a non-issue, predicated on two conventions.
The historical convention defines it as
$ Y_{\rm W} = 2(Q - T_3)$, as in the Gell-Mann—Nishijima formula of the strong interactions——for a conserved quantity. There, it was frequently used for strange particles, so the hypercharge could get to be 2, —2, etc... and a normalization like this one was warranted. In the weak interactions, thus, the weak hypercharge is defined as twice the average charge of a weak isomultiplet (where the average $T_3$ vanishes).
However, the more practical younger generation use $ Y_{\rm W} = (Q - T_3)$, instead, so the average charge of the isomultiplet, so, e.g., for right-handed fermions, weak isosinglets, the hypercharge is the charge, without daffy gratuitous 2s in the way. But it is only a matter of convention, and references such as the one you quote also specify the convention, as they should.
- Response to comment on conventions Recall both the Higgs entry in WP (Peskin & Shroeder conventions), and Srednicki's text are "modern", so the hypercharge is the average charge of the weak isomultiplet. Since, however, P&S put the v.e.v. downstairs in the Higgs doublet, that is the neutral component, so the upper one is charge +1, hence hypercharge 1/2. By contrast, Srednicki puts the v.e.v. upstairs, (87.4), so the lower component has charge -1, hence hypercharge -1/2. The averaging halves the units since one of the two components is neutral! A rule of thumb: to unconfuse yourself on such conventions, always, always , always , write down the Yukawa term that generates a mass for the charged lepton through its v.e.v. and monitor the charges and hypercharges of all fields, so the term conserves charge and hypercharge--as I'm sure you were trained to do in class.
