Hippeis and the Royal Bodyguard

While reading Herodotus the other day I came across the following passage:

"The prerogatives of the Spartan kings are these: ...they have a bodyguard of a hundred picked men...." (Herodotus, 6:56)

I stumbled over this because I, perhaps naively, had assumed up to now that the three hundred hippeis were the equivalent of the royal bodyguard. After all, according to other sources, "They were ... an infantry bodyguard for the king." (A.H.M. Jones, Sparta, New York, 1967, p. 63).  Or, Carledge describes them as "the crack royal bodyguard selected from the ten youngest age-classes." (Paul Cartledge, Sparta and Lakonia: A Regional History 1300 to 362 BC, London, 1979, p. 176.)

However, on re-reading Xenophon, I noted that he only describes how the three hundred hippeis are selected and the rivalry that Sparta encouraged among young men for the sake of winning the honor of such selection, but he does not describe their function, much less call them a royal bodyguard. 

There is also the question  of why, if each king was entitled to just 100 guardsmen, there were there three hundred hippeis?

Is it possible then that the royal bodyguards were something separate and apart from the hippeis? Did each king have a hand picked body-guard of 100 men, while the hippeis were appointed (indirectly) by the ephors and represented no a royal bodyguard but an elite unit under the control of the ephors?

There would be a certain logic to such a system. The hippeis would then represent the executive force and "bodyguard" of the elected representatives, i.e. the democratic elements of the Spartan state.  Perhaps the hippeis were even formed at a later date (as the ephors became more powerful) to counter-balance -- and out-balance -- the ancient tradition of royal bodyguards?

Any thoughts on this would be greatly appreciated!  I will be travelling next week and my next entry will not be until June 18.