It seems that the result of list of Timings and AbsoluteTimings depends on the order, while RepeatedTiming is free from this problem. How to understand this behaviour of timing functions?

The related codes are as follows. (The original aim is to compare the performance of similar functions on different types of data.)

Tested function

types = {"integer", "letter", "symbolLetter", "word", "symbolWord"}
functions = {complementVerbatim, Complement}
argnumber = 2;
(*`argnumber` specifies the number of arguments*)

complementVerbatim[list1_List, list2_List] := 
  DeleteCases[list1, Alternatives @@ Verbatim /@ list2];

Sample space

Choose portion of the samples as test data and save them into downvalues of data.

size = 2 10^4;
portion = 0.6;

sampleSpace["integer"] = RandomInteger[2 size, size];
sampleSpace["letter"] = RandomChoice[Alphabet[], size];
sampleSpace["word"] = RandomWord[size];
sampleSpace["symbolLetter"] = ToExpression /@ sampleSpace["letter"];
sampleSpace["symbolWord"] = ToExpression /@ sampleSpace["word"];

Table[data[type, argnumber] = 
   Table[RandomSample[sampleSpace[type], portion size // Floor], {i, argnumber}],
  {type, types}];

Timing

Now evaluate the functions on data with different order and timing functions.

Table[timing[Sequence @@ exception] := "/", {exception, exceptions}];

timing[function_, type_, argnumber, timingFunction_] :=
First@timingFunction[function @@ data[type, argnumber];]

timing[abs, fun] = 
  Outer[timing[#1, #2, argnumber, AbsoluteTiming] &, functions, types];
timing[abs, type] = 
  Outer[timing[#2, #1, argnumber, AbsoluteTiming] &, types, functions] // Transpose;

timing[re, fun] = 
  Outer[timing[#1, #2, argnumber, RepeatedTiming] &, functions, types];
timing[re, type] = 
  Outer[timing[#2, #1, argnumber, RepeatedTiming] &, types, functions] // Transpose;

timing[fun] = 
  Outer[timing[#1, #2, argnumber, Timing] &, functions, types];
timing[type] = 
  Outer[timing[#2, #1, argnumber, Timing] &, types, functions] // Transpose;

timingShow[x__] := SparseArray`SparseBlockMatrix[{
    {1, 1} -> {{""}},
    {1, 2} -> {types},
    {2, 1} -> List /@ functions,
    {2, 2} -> timing[x]
}] // TableForm

Result

Interestingly the three timing functions give different results.

The results of RepeatedTiming doesn'tdon't depend on the order:

while the results of Timings and AbsoluteTimings doesdo depend, and they are different from single runs like

timing[Complement, "integer", argnumber, Timing]
(*0.011819*)
(*comparing with 0.004266 in the table.*)

hence are untrustable.