Startup Manager is not offering the Recovery HD for booting

All that below is about os x 10.9.5 on iMac 2013 Late with SSD as the only internal drive and the mentioned os x installed on it and working without big problems.
At partition/disk level the ssd is as delivered by Apple, no adaptations were made by user at partition/logical disk level.
Merely the File Vault 2 was applied to this SSD.
That means amongst others no BootCamp.
Disk Utility presents for this SSD 4 items: Macintosh HD, EFI (grey color font), Macintosh HD (grey color font), Recovery HD (grey color font).

The Recovery Mode (by starting the mac while cmd+R is depressed) starts and seems to work pretty well.
However the Startup Manager (by starting the mac while the alt/option key depressed) does not offer the Recovery HD for booting.
It does offer merely the Macintosh HD for booting.
According to some article in web to which in this forum it is widely and frequently referring to,
the Startup Manager should also offer the Recovery HD on the system disk for booting.

I wonder why in case of this mac this does not happen?
Is this the valid and known design of 10.9.5 or the root cause to be searched where else?

I have a clone of that Recovery HD on some external USB storage.
The Startup Manager does offer this Recovery HD for booting very well.
Where the difference in handling these two Recovery HD's come from?

Thanks to Weaselboy for helpful information!

The conclusion for me is, all is fine with that Mac.
No more troubleshooting is needed. Valuable time can be
invested in tasks which really need it.

This is highly interesting.
I was namely - regarding this issue - a long time in contact with Apple.
A support ticket/case was open to clarify this issue.
Many weeks in discussion with senior level Apple support.
In the beginning the Apple support was informed the system disk is
protected by File Vault 2.
Lot of weeks of trials, tests and try-maybe-this checks.
No single word on side of Apple expert about the issue connections to FV2.
Finally the contact broke without single word of announcement.

Actually, I wonder why is this that way.
If a lack of ability to see partitions within big encrypted container is the reason
the system startup partition should not be visible for SM as well. FV2 makes whole disk encryption.
Is the property as you point out described in any Apple public document?

Hmm, I am not sure the article you are referring to is describes the behavior as
you described it here previously.
What is the source of your statement? Some Apple source of knowledge? Which one? Or rather all your experiences with Mac ?