Students repeat the experiment but replace block X and block Y with block W and block Z , as shown in Figure 3. Block W and block Z have identical mass M . When the experiment is conducted, the students observe that the blocks do not stick together, as shown in Figure 4. The students predict the speed of block Z after the collision by assuming that the collision is perfectly elastic. They then collect data about the actual speeds of both blocks immediately after the collision, as shown in the table.

Why does the predicted speed of block Z after the collision not agree with the actual speed of block Z after the collision?