Problem understanding System Verilog

Code Verilog - [expand]
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module getting_pro(
 input PB,
 input clk,
 output LED0,
 output LED1,
 output LED2
 );
 
 reg[2:0] counter;
//reflects state (pushed/not pushed)
 wire PB_state;
//is 1 for one clock cycle if button gets pressed
 wire PB_up;
//is 1 for one clock cycle if button gets released
 wire PB_down;
 
//debouncer module i found
 PushButton_Debouncer debouncer(
 .clk(clk),
 .PB(~PB),
 .PB_state(PB_state),
 .PB_up(PB_up),
 .PB_down(PB_down)
 );
 
//the not is because the LEDs are on when the Pin is 0
//The weired stuff happens here
assign LED0 = ~counter[0];
assign LED1 = ~counter[1];
assign LED2 = ~counter[2];
//if i change it to this
assign LED0 = ~counter[2];
assign LED1 = ~counter[1];
assign LED2 = ~counter[0];
//the leds blink as if counter[0] wouldn't exist 
// it counts up normally and resets normally it just starts with setting counter[1]
 
 always_ff @(posedge PB_down)
begin
  //if counter smaller than 7 count up , otherwise set counter to 0
   if(counter < 3'b111)
    begin
        counter <= counter + 3'b001;
    end
    else
    begin
        counter <= 3'b000;
    end
end
 endmodule