Mix.lock being rewritten with and then without "hexpm"

We’re getting similar churn in Mix.lock - changes that are just [repo: "hexpm", hex: :poison, optional: false] vs [hex: :poison, optional: false] (with/without repo: "hexpm") and with/without the trailing “hexpm”.

My colleague and I have both run mix local.hex, and both appear to already be on 0.17.1.

$: mix local.hex
Found existing entry: /Users/nathanl/.mix/archives/hex-0.17.1
Are you sure you want to replace it with "https://repo.hex.pm/installs/1.5.0/hex-0.17.1.ez"? [Yn]

Any other reason why this might happen?

Here’s an example diff.

-  "phoenix": {:hex, :phoenix, "1.3.0", "1c01124caa1b4a7af46f2050ff11b267baa3edb441b45dbf243e979cd4c5891b", [:mix], [{:cowboy, "~> 1.0", [repo: "hexpm", hex: :cowboy, optional: true]}, {:phoenix_pubsub, "~> 1.0", [repo: "hexpm", hex: :phoenix_pubsub, optional: false]}, {:plug, "~> 1.3.3 or ~> 1.4", [repo: "hexpm", hex: :plug, optional: false]}, {:poison, "~> 2.2 or ~> 3.0", [repo: "hexpm", hex: :poison, optional: false]}], "hexpm"},
-  "phoenix_ecto": {:hex, :phoenix_ecto, "3.3.0", "702f6e164512853d29f9d20763493f2b3bcfcb44f118af2bc37bb95d0801b480", [:mix], [{:ecto, "~> 2.1", [repo: "hexpm", hex: :ecto, optional: false]}, {:phoenix_html, "~> 2.9", [repo: "hexpm", hex: :phoenix_html, optional: true]}, {:plug, "~> 1.0", [repo: "hexpm", hex: :plug, optional: false]}], "hexpm"},
+  "phoenix": {:hex, :phoenix, "1.3.0", "1c01124caa1b4a7af46f2050ff11b267baa3edb441b45dbf243e979cd4c5891b", [:mix], [{:cowboy, "~> 1.0", [hex: :cowboy, optional: true]}, {:phoenix_pubsub, "~> 1.0", [hex: :phoenix_pubsub, optional: false]}, {:plug, "~> 1.3.3 or ~> 1.4", [hex: :plug, optional: false]}, {:poison, "~> 2.2 or ~> 3.0", [hex: :poison, optional: false]}]},
+  "phoenix_ecto": {:hex, :phoenix_ecto, "3.3.0", "702f6e164512853d29f9d20763493f2b3bcfcb44f118af2bc37bb95d0801b480", [:mix], [{:ecto, "~> 2.1", [hex: :ecto, optional: false]}, {:phoenix_html, "~> 2.9", [hex: :phoenix_html, optional: true]}, {:plug, "~> 1.0", [hex: :plug, optional: false]}]},