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Simpley because some of the elements you have used have default margins.
Employ a standard reset and all is good.
https://codepen.io/Paulie-D/pen/zebBKE?fbclid=IwAR2xCtwQsG3tfi1X715ATWUpfDMCBjwkXDBfkQfUSQRHbeK8d1sJJ9-SoOY
