It's not strange. It goes by population.
Hillary landslides the important states and counties (ie with the highest population and delegate count), which means he can win a lot of those tiny states and they would have less combined delegates than the biggest county that Hillary wins. It's pretty much the problem with Sanders, he does extremely poor in the important states and counties.
Looking at tonight, Hillary will extend her lead to 300 delegates. Going by mathematics, if Bernie does not do well next week, his campaign will be over... he needs to win all remaining states by 61%. Knowing that Hillary polls 60+% in a lot of the upcoming states, means that Sanders will likely not get any victories, and if he does, it will be miserable victories, that will keep Hillary ahead.