Why does Default behave like this?

One may set a Default value for the arguments of a function:

Default[f] = 5;

And then use:

f[a_, b_.] := {a, b}

f[1, 2]
f[1]

   {1, 2}

   {1, 5}

This creates the following Values:

DefaultValues[f]
DownValues[f]

   {HoldPattern[Default[f]] :> 5}

   {HoldPattern[f[a_, b_.]] :> {a, b}}

From this one might think that the value 5 is not fixed in the definition of f, but addresses the DefaultValues assignment. However, if we change the DefaultValues, either directly or using:

Default[f] = 9;

DefaultValues[f]

   {HoldPattern[Default[f]] :> 9}

and use f again:

f[1]

   {1, 5}

we see that the new value is not used.

Therefore, my questions are:

• Why does the default value used by f[a_, b_.] := {a, b} not change with DefaultValues?

• Where is the real default value (5) stored, since it does not appear in either DownValues or DefaultValues?