Mark, don't be mislead. It's fine to measure by volume when appropriate. After all, most people probably measure their water, or at least the final mix size, by volume.
Originally Posted by albada
Your discrepancy just tells me that either your syringe or the MSDS statement of specific gravity probably isn't all that accurate. Maybe even your scale. Without some way to crosscheck your gear, it's hard to know exactly which part is off.
Even so, your ~3% discrepancy is probably not too big a deal. (It would be nice to know the significance, though.)