Hi,
Each unit is dropped with a probability 0.3. In your case, you have 10 values.  You may expect exactly 3 nodes to be dropped every time. It does not need to be exactly 3. Sometimes, it might be 2. Sometimes it can be 4 or even 5. I can one intuitive example.
Suppose you have an unbiased coin. P(H)=P(T)=0.5 (Here H-Head, T-Tail). If you toss the coin 10 times, on average you get 5 Heads and 5 Tails. But it can be 4 Heads and 6 Tails. The probability enforces that the expected number of heads in 10 tosses is 5.
Thanks
Regards
Pranavan