oo::class - my variable vs variable

I am trying to learn TclOO. I have created two classes P and Q which
appear to have identical behavior. Class P uses "my variable" and Q uses
"variable". Can someone explain the difference between them. (I am
familiar with Python and Go if that's any help with explaining.)

oo::class create P {
    constructor {{x 0} {y 0}} {
        my variable x_
        my variable y_
        set x_ $x
        set y_ $y
    }

    method x {} {
        my variable x_
        return $x_
    }

    method set_x {x} {
        if {![string is integer -strict $x]} {
            throw NOT_AN_INT "x must be an int not \"$x\""
        }
        my variable x_
        set x_ $x
    }

    method y {} {
        my variable y_
        return $y_
    }
}

oo::class create Q {
    constructor {{x 0} {y 0}} {
        variable x_
        variable y_
        set x_ $x
        set y_ $y
    }

    method x {} {
        variable x_
        return $x_
    }

    method set_x {x} {
        if {![string is integer -strict $x]} {
            throw NOT_AN_INT "x must be an int not \"$x\""
        }
        variable x_
        set x_ $x
    }

    method y {} {
        variable y_
        return $y_
    }
}

puts "P"
set p1 [P new]
puts "p1 x=[$p1 x] y=[$p1 y]"
$p1 set_x 5
puts "p1 x=[$p1 x] y=[$p1 y]"
try {$p1 set_x "invalid"} trap {} err { puts $err }
set p2 [P new 0 -8]
puts "p2 x=[$p2 x] y=[$p2 y]"
$p2 set_x 17
puts "p2 x=[$p2 x] y=[$p2 y]"
puts "p1 x=[$p1 x] y=[$p1 y]"

puts "Q"
set q1 [Q new]
puts "q1 x=[$q1 x] y=[$q1 y]"
$q1 set_x 5
puts "q1 x=[$q1 x] y=[$q1 y]"
try {$q1 set_x "invalid"} trap {} err { puts $err }
set q2 [Q new 0 -8]
puts "q2 x=[$q2 x] y=[$q2 y]"
$q2 set_x 17
puts "q2 x=[$q2 x] y=[$q2 y]"
puts "q1 x=[$q1 x] y=[$q1 y]"