Bear in mind that you're doing different things here. If you allocate four cores from one slot, you're allocating four cores and four cores worth of memory (the exact amount depends on your hardware to the guest. If you allocate one core per slot you are allocating one core to the guest, and setting up three additional slots to help with the traffic.
You want to ensure you have enough cores per slot allocated that if you lose a blade, the remaining blades can handle your full traffic load. Each slot essentially acts as a full guest handling a portion of the incoming traffic under the direction of the primary slot. So this is completely expected.
This is detailed here: https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/vcmp-administration-viprion-13-1-0/3.htmlguid-f13cc385-46b0-40ac-b695-871d7791a1fe
To reiterate: Cores allocated on a single slot increase the cores and memory available to that guest. Cores allocated on additional slots increase the number of slots available to the guest. Both will increase the ability to handle traffic, but in different ways.