Straight lines with ruler sometimes thickness various and points off

i've noticed this for around 3 years now and it happens quite often, if i draw a line with the vector brush (no changes to brush standard round brush settings) with the ruler, i can see the line thickness varies & isnt consistant thickness , then at the end of the drawn line "tails off" and goes to a point.  

it usally starts happening once youve used the brush & ruler  a few times to draw straight lines..

ios app.