Umicore is an LBMA member and you'll generally find that companies who work to that sort of standard will err on the side of caution and produce slightly over-weight bars rather than under-weight ones.
Depending on how accurate your scales are, it could easily due to rounding on a fractionally over-weight bar as suggested above. If your scales are out by 0.25g and the bar is over-weight by 0.25g then that would add up to 0.5g and subsequently get rounded up to 1g.