On the face of it this would seem a plausible and reasonable explanation.
It still leaves you with what I consider a sub standard warranty though.
Perhaps you should get your solicitor to ensure that your warranty, whilst with Premier Guarantee, is a like for like policy with the NHBC Buildmark warranty, even if the standard Premier warranty is, as I suspect, not as good.
And through error or not, you have been misled by the builder.
Some from of compensation would be warranted, even if it is only turfing your rear garden for free.