Why are these using swap when I have plenty of free ram?

[Chris]

The below processes are using swap for some reason:

User     Name                         Swap
root     courierlogger                  76
daemon   atd                           188
root     acpid                         116
avahi    avahi-daemon                  300
root     lvmetad                       208
root     authdaemond                    12
root     osspd                         124
root     cron                          184
root     wpa_supplicant                480
systemd-timesync systemd-timesy        460
systemd-resolve systemd-resolv         492
root     irqbalance                     44
avahi    avahi-daemon                  220
root     systemd-logind                424
root     smartd                        564
root     accounts-daemon               312
syslog   rsyslogd                      404 
gdm      (sd-pam                      1440
root     systemd-udevd                1036
messagebus dbus-daemon                 252
chris    (sd-pam                       188
root     systemd                       196
root     networkd-dispat              3344
root     udisksd                       252
root     systemd-journal               340
  
 total        used        free      shared  buff/cache   available
Mem15351148  6475100     3012432    407808   5863616     5408332
Swap:       4089852       53248     4036604

Before anyone asks I've had a swap partition for years dating way back
so I've just never removed it. It's only been a month or so that swap
has started being used and I have no idea why with plenty of free ram.
Of course it's not hurting anything however I'm really curious as to
why these processes are using it. Ubuntu version and kernel version are
in my sig.

-- 
Chris
31.11972; -97.90167 (Elev. 1092 ft)
15:32:17 up 9 days, 1:10, 1 user, load average: 0.72, 1.09, 0.92
Description:	Ubuntu 18.04.3 LTS, kernel 5.0.0-32-generic