Robertson-Macdonald confusion!

Question

Here's quite a distinctive hyphenated family name:  Robertson-Macdonald

For example, the Admiral (of Kinlochmoidart)

[[Robertson-Macdonald-4|David Robertson-Macdonald (1817-1910)]]

I've found what looks like another one that was following in the family military tradition.

[[Robertson-MacDonald-10|Alan Robertson (Robertson-MacDonald) MacDonald (abt.1896-)]]

As a cadet at the Dartmouth Naval College.

And Alan's father:

[[Robertson-Macdonald-11|David S Robertson-Macdonald (abt.1926-)]]

But can't figure-out the connections to the Admiral or older members of the hyphenated family.

Any ideas?