Why did this delta wing generate a lower Cl at a higher wind tunnel velocity?
The increase in the lift curve slope at low angle of attack is quite normal. At 10° you see the effect of vortex lift kicking in. Yes, that is specific to delta wings.



Attached and vortex lift over angle of attack (picture source). This is taken from the excellent Cambridge Core article on vortex lift and these particular diagrams are from E.C. Polhamus: A concept of the vortex lift of sharp-edged delta wings based on a leading-edge suction analogy, NASA TN D-3767, December 1966. They show lift over angle of attack, but lift over drag would look quite similar, also with a kink once vortex lift sets in. 

The offset between the two speeds is harder to explain. This looks like an error in your angle measurement – there must have been different offsets between the two test runs. That there is an offset is obvious from the nonzero lift at zero angle of attack. Unless your delta wing had lots of camber, there should not be much lift at zero angle of attack.