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The installer has used an aluminium plate as the main
earthing conductor, probably due to it's 'resistance to corrosion'.
By using brass earthing bolts, and being in an ideally damp location,
he has created a battery, in which the aluminium will freely erode.
This is a good example of 'galvanic corrosion', familiar to many boat
owners. The brass is a less reactive (more 'noble') metal, and the aluminium
acts as a 'sacrificial' anode.
The earth conductor will, over time, increase in resistance due to build
up of aluminium oxide, until it no longer fulfils it's protective function
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