[metapost] arrowhead weirdness

[Oliver Buerschaper]

Dear MetaPosters,

while experimenting with arrow heads I've stumbled upon something that  
puzzles me:

---
u := 0.5cm;
path p;
p := origin--(5u,0);
ahlength := 2u/sind(ahangle);
pickup pencircle scaled u;
drawarrow p;
draw p yshifted u withpen pencircle scaled 0.1bp withcolor red;
draw p yshifted u/2 withpen pencircle scaled 0.1bp withcolor red;
undraw p withpen pencircle scaled 0.1bp;
undraw arrowhead p withpen pencircle scaled 0.1bp;
---

Shouldn't the upper corner of the white arrowhead path lie on the red  
line? Would be grateful if someone could point me to what I'm missing …

Thanks in advance,
Oliver