Complete binary tree property has this property that every level until last is fully filled and last level is filled from left to right.
So, when we have a complete binary tree with $7$ nodes,
- $\text{level} _1$ has $1$ node ( root )
- $\text{level} _2$ has $2$ nodes.
- $\text{level} _3$ has $4$ nodes.
BFS goes level by level. So first three elements (say Set A) $=\text{level} _1$ nodes $(1)+ \text{ level} _2(2)$ nodes.
DFS is go by connected manner. So first three elements (say Set B ) $= \text{level} _1$ nodes (1)$+$ one node from $\text{level} _2$ nodes $+$ one node from $\text{level} _3$ nodes which is connected to the previously chosen $\text{level} _2$ node.
$A – B =$ the remaining node from the set of $\text{level} _2$ nodes.
$\implies |A – B| = 1.$