what does the P_i mean?

[LinRds]

Hello there, i am confused with the definition of $P_i$，the formula $P_i = \sum_{x \in C_i}P_L(x)$ indicates that it is the sum of the probability of all labeled example which belong to $C_i$.

For examle in (b), the two red point in the bottom right corner are belong to same component $C_3$, so $P_3 = P_L(red_1) + P_L(red_2)$.However it can't guarantee $P_3 \le 1$，so i wonder how to understand the definition of $P_i$.