My guess 737ng is .....that at FL 330 the ac could already be in cruise phase- because of probably being within 0.25 psid of the FLT ALT selection; FL 350 in your example. Now resetting FLT ALT to 330 may thus initiate a slight cabin descent.
As per the MRG ....refer pressurisation schedule.....
"This table explains why the cabin rate of climb indicator shows a descent when the airplane makes a step climb from a level flight below FL370 to a (new) cruising level above FL 370."