I asked one of the guys I work with, he thought that it might be an attempt
to achieve impeadance matching: there may be an inner layer with signals
that line up with these traces, possibly with internal vias under the larger
copper-grouping areas.

Also note, the many separate copper traces would have lower capacitance with
an internal layer than a solid/wide ground plane would...

Taken a step further: He mentioned that he knows of one company that
actually makes coax-equivelent shielded traces by having enclosing traces on
the two adjacent layers and laser drilled plated micro-holes bonding the
enclosing layers.

Bruce.

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