Impossible newline handling? Can't do just one newline?

Can someone tell me why this forum strips the newline between lines when Shift+Enter is used, but does not strip any newlines at all when just Enter is hit or when Shift+Enter is hit twice in a row?

Examples:

line 1 then Enter line 2 after Enter

line 3 after two Enters

line 4 after another two Enters

line 5 after two Enters but before one Shift+Enter line 6 after one Shift+Enter

line 7 after two Enters before two Shift+Enters

line 8 after two Shift+Enters