too many matches regex

$s = '1 2 20 - 31 55 56 77 - 88 97 - 99';

$patRange = '/((\d+) *- *(\d+))/';

$matches = array();
preg_match_all($patRange, $s, $matches);

I expected to see an array with 3 matches for each of the there number
"ranges" found in the
string $s (20 - 31, 20 and 31).... I see 4. Why? Need some insight
from a jones. I don't understand what I'm seeing.

Thank you.

Array (PREG_SET_ORDER)
(
[0] => Array
(
[0] => 20 - 31
[1] => 20 - 31
[2] => 20
[3] => 31
)

[1] => Array
(
[0] => 77 - 88
[1] => 77 - 88
[2] => 77
[3] => 88
)

[2] => Array
(
[0] => 97 - 99
[1] => 97 - 99
[2] => 97
[3] => 99
)

)

Array (PREG_PATTERN_ORDER)
(
[0] => Array
(
[0] => 20 - 31
[1] => 77 - 88
[2] => 97 - 99
)

[1] => Array
(
[0] => 20 - 31
[1] => 77 - 88
[2] => 97 - 99
)

[2] => Array
(
[0] => 20
[1] => 77
[2] => 97
)

[3] => Array
(
[0] => 31
[1] => 88
[2] => 99
)

)

Array (null)
(
[0] => Array
(
[0] => 20 - 31
[1] => 77 - 88
[2] => 97 - 99
)

[1] => Array
(
[0] => 20 - 31
[1] => 77 - 88
[2] => 97 - 99
)

[2] => Array
(
[0] => 20
[1] => 77
[2] => 97
)

[3] => Array
(
[0] => 31
[1] => 88
[2] => 99
)

)
.