You only get one message as they starve to prevent a log overload.
When a planet starts starving a log entry is place in the players log. If the planet is still starving the next tick (5 minutes) the number of colonists that starved this tick is added to the number from last time. Then the log entry tade is change to the current date so the log entry is at top.
So what you are seeing is the total number of colonists that have starved since the planet started starving.
I guess we need to add a start starve date to the log entry so you know when they started starving. That way you can feel bad for them when you realize they have been starving for days.