וְלֹא יִשָּׁפֵךְ דָּם נָקִי בְּקֶרֶב אַרְצְךָ
And 21:8:
וְאַל־תִּתֵּן דָּם נָקִי בְּקֶרֶב עַמְּךָ יִשְׂרָאֵל
And then Ki Savo 27:25:
לְהַכּוֹת נֶפֶשׁ דָּם נָקִי
However, back in Shof'tim, 19:13:
וּבִעַרְתָּ דַם־הַנָּקִי מִיִּשְׂרָאֵל
… which uses the definite variant of "דַּם־נָקִי", not of "דָּם נָקִי".
Why is "דָּם נָקִי" (literally "innocent blood") used in most places and (a form of) "דַּם־נָקִי" (literally "blood of an innocent") used in 19:13?
Is there a difference in meaning between "דָּם נָקִי" and "דַּם־נָקִי"?
(I'm asking about "דָּם נָקִי" vs. "דַּם־נָקִי", not about definite vs. indefinite.)