Forgive me if I lecture, that's not my intent. I only want to put
these thoughts into words so I can refer back to them someday when
this comes up again.

I believe what Turbine is saying is true, but not in the way people
think it is.

There are two types of xp, Character xp and Skill point xp. This is
the basis for the misunderstanding.

Character xp is what we refer to as "spendable". Its the xp we get
when we kill stuff. Whenever I kill a Tusker Slave, I get 11,000
character xp regardless how many I've killed in the last 10 minutes.
If this number went down, it would be 0xp by the time I finished
clearing the BSD level I'm on.

There's also Skill Point xp, the basis for macroing characters. You
gain a certain amount of xp based on passing a "skill check" based on
a certain amount of time since you last passed the same skill check
(usually 10 minutes).

Skill checks differ for the various skills, obviously. But they are
generally follow a "level" method for granting xp. Casting an Armor
III and Lightning Pro III are the same level spell in the same
school, so you will only get skill xp for the first spell. If you
waited 10 minutes then cast the second spell, you'd get full skill
check xp for that casting.

Here's the way it works for me. I'm a swordsman in the BDC.

I attack a Tusker Slave. First time I hit him (evasions are failed
skill checks), I get a certain amount of xp towards my sword skill,
and a small amount of this (usually around 10%) as
spendable/character xp. Now we know I'm not one-hitting tuskers
(yet? :) so the second time I hit him I get 0 sword skill xp. Say I
kill this tusker. I get 11,000 character xp as spendable. So, say I
got 100 sword skill xp (I dunno what it really is), so my total xp on
this first tusker is 11,100.

Now the second slave has started beating on me. Uh-oh I better kill
it, too. So I swing and connect, but since its been too soon since I
passed my last sword skill check ON TUSKER SLAVES I get no skill xp.
I kill this second tusker and I get another 11,000 xp, 100 less than
the first one.

Suddenly from out of nowhere a Tusker Guard rushes up and almost one
hits me (because I'm sitting here with a calculator instead of paying
attention of course), so I have to kill it as well. I swing and
miss, swing and miss, swing and finally connect. I see that instead
of 100, I get 200 xp in my sword skill. Why is this? Its a new
creature, of a higher level so its a new skill check, which I've
passed. I kill this guard (close one, tho) and notice I get 13,500
xp in my character xp total. Total xp for the guard is 13,700.

If another guard ran up and I killed it, I would get 13,500.

The skill check numbers are approximate, and frankly they are more
pronounced for mages than they are for warriors because of the
different schools being used and different levels but the principles
are the same.

So as you see, if you just keep whacking the same thing over and
over, you will get "less" xp than if you ran around killing lotsa
different stuff because your skill checks will not reset as often.
But you don't get diminished amounts of character xp for killing the
same monsters over and over.

At least not in my experience.

I will be the first to say nobody from Turbine has come out and
said, "Yes Phil, this is the way it works". What I've just stated is
based on everything I've read in the strat manuals, dev boards/chats
and my own extensive experimentation (did you know you don't get
skill xp for casting certain spells? Portal spells, blood loather,
etc. That's gotta be a bug) so feel free to take all this with a
grain of salt. Do I know all this to be "fact"?

Let's just say I'm 99.99% certain of all of it.

Please let me know if your experience (pun intended) differs.