change hystersis in schmitt tirgger with transistor

hi
someone can help me to analysis this circuit?
when Rpot is 1k and I start to reduce pot value the relay change 
condition((ON)) when Rpot= 275ohm and when I increase Rpot relay 
comeback unchange mode((off)) in 500ohm ,this operation it's okay to me 
but if I put Rpot = 470 ohm and start to reduce the Rpot the relay is on 
mode why???
and why does designer put Re2,Rb1,Rb2 ??? when I pick up Re2 my 
hysteresis change??