Calling Matlab/Psychtoolbox from the Shell or from a Makefile does not give the same behavior ! Why?

Question

I thought a Makefile was just executing the stated shell commands, but things seem not that simple : sample.m is a minimal matlab program that displays a word on screen. When launched from a Makefile, it does not behave the same as when launched from the shell...

From the shell

alex:~$ matlab -nosplash -nodisplay -r "sample"

-> Displays the word correctly

From a Makefile

all :  
matlab -nosplash -nodisplay -r "sample"  

alex:~$ make

-> Displays the word with a blue bounding box

How can it be different ? I'm using Matlab 2010a on a Ubuntu 10.04 machine. No arguments are passed to the sample.m script.