electrophoretic mobility

[C.S. Wilding]

This is probably one of those things that I should know the answer to 
or be able to apply myself to but I don't and can't so:
When electrophoresing (through 0.8% agarose in 1xTBE) some restriction 
digested DNA samples, why do samples cut with EcoRI in NEB's Eco buffer 
(50mM NaCl, 100mM Tris-HCl, 10mM MgCl2, 0.025% Triton X-100) "appear to 
run slower" than uncut DNA or Eco cut in NEB buffer#2 (10mM Tris-HCl, 
10mM MgCl2, 50mM NaCl, 1mM DTT). I say "appear to run slower" as this 
is based solely on the migration of the bromophenol blue and xylene 
cyanol which is equal in uncut (no buffer) and #2, but much slower in 
those with the Eco-specific buffer.
Craig.