I agree about the platelets. Something
is helping in that regard.
Anemia (decrease in RBCs) can be caused by internal bleeding or by destruction of red blood cells. My guess is that the vet is thinking there's a hidden cancer that's causing internal bleeding
How far out of norm is the WBC count? White blood cells come in a number of varieties--some WBCs are more commonly seen when the body is fighting a parasite, some when there is a bacterial infection, some when there is allergy-related inflammation, some when the body is fighting cancer, etc. As MaxLisa mentioned, sometimes the pattern of WBC activity can tell you something about what the problem is...
Your vet should be willing to sit down and talk to you (or talk over the phone) about what his suspicions are and why he thinks that way.
Did Timber eat anything tonight?