When I tried to combine on one line, while no syntax errors, only the Add worked.
Set-SendConnector -Identity "sendConnTest" -smartHosts @{Add="192.168.150.151", "192.168.150.152"}; @{Remove="192.168.150.150"}
So I had to do this as two separate lines:
Set-SendConnector -Identity "sendConnTest" -smartHosts @{Add="192.168.150.151", "192.168.150.152"}
Set-SendConnector -Identity "sendConnTest" -smartHosts @{Remove="192.168.150.150"}
But to answer my question, 'SmartHostsString' was an automatically updated attribute.
SmartHosts : {[192.168.150.151], [192.168.150.152]}
SmartHostsString : [192.168.150.151],[192.168.150.152]
Hope this helps others:)