You may notice the same thing next time you are on an E170/190. I suspect for the same reason, but I'll let the Bus guys chime in first.
Really?
The minimum flex takeoff thrust is limited to 75% maximum rated takeoff
thrust or CLB-2 + 1% N1, whichever is higher.
and
Whenever
the selected takeoff thrust is lower than CLB-1 the CLB-2 mode becomes
the default until the next airplane power down / power up.
Manual switching between the climb modes is possible anytime in flight
on the MCDU - TRS page.
On ground the CLB-1 mode is inhibited if the take off thrust selected is
lower than CLB-1 thrust.
Would suggest not. :-)