Thank you for the opportunity to help you with your question!
Its easier because you're doing the work in smaller steps. Eachstep requires less energy, and even though it will take longer toclimb via zigzag, you have more time to recover some of the lostenergy. Its just like its easier to move a ton of bricks via manylittle trips than one big one. Even though it requires the same amount of energy, its spread out.
Please let me know if you need any clarification. I'm always happy to answer your questions.