question on elapsed time

I am working a problem that require to understand why each operation takes different time. So, I have a

A = randn(100,100);
b = randn(100,1);
tic; x = A\b; toc;

this operation takes about 0.002s

then I do

tic; x= A\b; toc;

MatLab takes slightly shorter times to do the calculation.

As I repeat for a few times, I keep getting two slightly different time. I don't understand why.

Answer 1

[Star Strider]

You are getting slightly different times because life is inherently stochastic.

Answer 2

[Walter Roberson]

• You wiggled the mouse more and that took up some of the computer's attention
• MATLAB does not invoke its garbage collection every time it does an operation, and the amount of time it takes to collect garbage is not constant
• Your anti-virus kicked in for some reason
• Your firewall spent a bit of time disregarding some attack packets from outside
• Some background operation resulted in the data being moved out of the Level 1 cache of your CPU
• Your operating system decided to schedule the work for a different processor and spent a moment moving the process
• Your memory system spent a few jiffy fixing a single bit error
• Your music player decided it was time to check with the mothership to see if a new version of itself was available
• Sunspots
• Because the wise and powerful and rich and wise weasel wished it so