a question about why implicit rule must use the path found by directory search!!

[ali hagigat]

/root> touch di4/preq1.u
---------------------------------
vpath preq1.u di4
all6: preq1.u
        @echo "kkkkk"
%.u: preq2
        @echo "hhhhh"
        @touch $@
preq2:
        @echo "llllll"
---------------------------------
The manual says:
4.4.5 Directory Search and Implicit Rules
The recipes of implicit rules normally use automatic variables as a
matter of necessity;
consequently they will use the file names found by directory search
with no extra effort.
----------------------------------
In the example I wrote, why 'di4/preq1.u' is not touched by '@touch $@'?
Why the pathname found by directory search means 'di4/preq1.u' was not
used by the automatic variable of our implicit rule($@)?