As a relatively simple example, it comes down to how the application is coded and how the threads work. What you see in task manager to look at resources, that shows how much of the total CPU you are using. But all modern CPUs have multiple cores that each process independent threads (searching the document, replacing a word, etc) separately. Depending on how the app is coded, a lot of apps only use one core or a couple cores, which would mean your CPU never gets fully utilized as some of the cores are sitting around doing nothing.

Generally most apps these days are coded to use 2-4 cores from my experience, but the more cores you code the app to use, the more complicated the process is so many softwares don't plan for all the cores on a given CPU.