php function implementation overrides non-nullable argument

I ran into this today with PHP (7.1 and 7.2 at least) with the following code:

namespace PlaceHolderX\Tests\PHPUnit\Unit;

use PHPUnit\Framework\TestCase;

final class BreakingClassesTest extends TestCase
{

    public function testBreak(): void
    {
        $tester = new SomeClassA();
        $tester->test();
        $this->assertNull($tester->get());
    }

}

interface InterfaceA {

    public function test(string $testString): void;

}

class SomeClassA implements InterfaceA
{
    /** @var null|string */
    private $testString;

    public function test(string $testString = null): void
    {
        $this->testString = $testString;
    }

    public function get(): ?string
    {
        return $this->testString;
    }
}

So I have an interface (InterfaceA) that has a method that requires a string. This argument is not nullable, cause if I wanted that I would have specified it as:

public function test(?string $testString): void;

But in the implementation class (SomeClassA) I can override the argument definition with a default value of null which results in a behavior I didn't intend with my interface.

So my main question is: Why is this possible? Of course, we will need to check this in code reviews, but it is something that is easy to miss.

I tried searching what causes this behavior but was not able to find an explanation. Maybe my search criteria are off.