set and sorted, not working how expected!

[wtr]

Hello all,

I implemented the following. I don't understand why the output is unsorted list. I'm using set and sort to filter my list.
freq_list = set(sorted(i.f_center for i in ret))

However breaking down the steps

1. i.f_center for i in ret # extracts a list of frequency values. See below
   

a = [1225000000.0, 5800000000.0, 434000000.0, 2450000000.0, 2425000000.0, 5750000000.0, 455000000.0, 441000000.0, 441000000.0, 5850000000.0, 455000000.0, 1225000000.0, 5740000000.0, 5800000000.0, 1200000000.0, 5820000000.0, 441000000.0, 2475000000.0, 5850000000.0, 915000000.0, 2585000000.0, 5740000000.0, 2425000000.0, 5750000000.0, 1165000000.0, 1225000000.0, 915000000.0, 434000000.0, 872000000.0, 5800000000.0, 2415000000.0, 2415000000.0, 2425000000.0, 1105000000.0, 872000000.0, 5750000000.0, 434000000.0, 2450000000.0, 2475000000.0, 455000000.0, 1255000000.0, 1105000000.0, 1255000000.0, 2585000000.0, 5870000000.0, 915000000.0, 2585000000.0, 1165000000.0, 2450000000.0, 5820000000.0, 5870000000.0, 5740000000.0, 5870000000.0, 5820000000.0, 5780000000.0, 2475000000.0, 2415000000.0, 1105000000.0, 5850000000.0, 1200000000.0, 5780000000.0, 1200000000.0, 1165000000.0, 5780000000.0, 872000000.0, 1255000000.0]

[list=2]
[*]now apply the sorted and set functions.
[/list]

a1 = sorted(a)
print(a1)
a2 = set(a1)
print(a2)

You'll notice that 441000000.0, is not where I expect it.