They may have pine and just don't know it.
Most coniferous graded lumber is stamped S-P-F , for spruce-pine-fir they are all grouped together.
S-P-F includes balsam fir , all spruce , all pine , hemlock , and larch (or tamarack to you Americans).
Douglas fir is separated.
You can tell the fir apart as it is usually quite red , as for spruce and pine , look for the S-P-F stamp , and buy the boards with the smallest and fewest knots , chances are , it is spruce and not pine , as pine usually has more and larger knots , even then , it's hard to be sure.
They are so close though , I don't think it would really matter.