[ubuntu_mate] Installer wants to put extra partitions on a new SSD

I'm trying to install Ubuntu-MATE 18.04.4 on a new, empty, external standalone 1 terabyte SSD.

I have 2 other drives in my desktop workstation, so in the installer I selected "do not use" for the swap spaces on those drives.

I then selected to create an 8 GB swap space for the SSD as a "logical" or extended partition at the end of the SSD's empty space. (Since my desktop box has 8 GB of RAM.)

Then I set up one large primary partition for both "/" (root) and /home directories from "Beginning of space" using the ext4 filesystem.

I selected for the bootloader to be on "sde" which is the SSD.

But when I click on "Install now," the informational box that pops up says that the large primary partition will be partition 2, and that the 8 GB swap partition will be partition 5.

The installer wants partitions 1, 3 and 4 to be "unallocated space" partitions. Is this normal? I defined just 2 partitions. I don't understand why the installer wants to create five partitions on my SSD.

Since I want to understand what's going on with this, I did not go ahead with the install.

Can anyone shed light on what's going on here?