Hi tigmeister, the grades for those deposits are lower because...

Hi tigmeister, the grades for those deposits are lower because the ore body is slightly different and they used a lower cut-off grade, given that the deposit is close enough to the surface to allow for an open-pit bulk tonnage operation. This method is much cheaper than underground mining, making lower-grade gold economical compared to narrow vein mining.

This doesn’t mean there aren’t high-grade veins within the ore body which is typical of the area. However, since there are larger thicker intercepts of lower-grade gold surrounding it, it makes sense to change the mining method and mine the entire ore body. This is why they publish a larger tonnage but lower-grade resource.

For example, drilling in 2008, HGD12 at Hargraves hit 75m @ 4.2g/t from 22m, and this included high-grade veins such as 610g/t over 0.18m and 722g/t over 0.2m, similar to the Reward veins. Other nearby hits include 35m @ 2.6g/t and 102m @ 1.5g/t, located 100m to the south, indicating decent continuity of the ore body.

Some comments about HEG's drilling of the above mentioned hits are below:
https://announcements.asx.com.au/asxpdf/20080902/pdf/31c2nv7qc9ms9r.pdf