I dont understand the output of diff function from difflib module

Question

Im having a hard time understanding the output of the Difflib module of python. This output seems fine here:

import difflib
from difflib import SequenceMatcher, Differ
diff = Differ()

test_string_1 = "Ava D Ava: I'll try... :)"
test_string_2 = "AvaDAva:I'lltry...:)"
diffs = list(diff.compare(test_string, test_string_2))
print(diffs)

[
    "  A",
    "  v",
    "  a",
    "-  ",
    "- :",
    "  D",
    "-  ",
    "  A",
    "  v",
    "  a",
    "  :",
    "-  ",
    "  I",
    "  '",
    "  l",
    "  l",
    "-  ",
    "  t",
    "  r",
    "  y",
    "  .",
    "  .",
    "  .",
    "-  ",
    "  :",
    "  )",
]

But when add a colon before the D character the output changes drastically:

test_string_1 = "Ava :D Ava: I'll try... :)"
test_string_2 = "Ava:DAva:I'lltry...:)"
diffs = list(diff.compare(test_string, test_string_2))
['- A',
 '- v',
 '- a',
 '-  ',
 '- :',
 '- D',
 '-  ',
 '  A',
 '  v',
 '  a',
 '  :',
 '-  ',
 '+ D',
 '+ A',
 '+ v',
 '+ a',
 '+ :',
 '  I',
 "  '",
 '  l',
 '  l',
 '-  ',
 '  t',
 '  r',
 '  y',
 '  .',
 '  .',
 '  .',
 '-  ',
 '  :',
 '  )']

The output suggests both strings start differently when they are the same. I really need help, this doesn't make sense to me.