Impact of partition scan on large and small data
Think of it as different queries. How much it cost to navigate a B-Tree to find zero rows? That is what happens for those "extra partitions". Those that don't contain any data for your range and which are skipped with your second query alternative. If you don't have too many partitions, then this extra cost will be very low. But as number of partitions increase, so will the overhead (especially relative overhead if the selectivity for the range is high).