e out more like 60%.
span 2 80% Data
Data span 2 80%

With the columns the opposite way round it comes out even worse!
Data span 2 80%
span 2 80% Data

Three cols all meant to be 40% (last comes out as 20!):
40% 40% 40%

Five cols all meant to be 40% (last two come out as minimum and whole table unnecessarily stretched):
40% 40% 40% 40% 40%

A table with abs widths which are too small - cells overspill
20 ie not-big-enough 10 even-worse

An absolute constraint and a percentage one (both met)
400 10%

Yikes
z zz
zz zz
zzz
z z z
50% without quotes weeble weeble weeble dull
50% weeble weeble weeble dull
relative 3* weeble weeble weeble dull
col 2* col 100
dull
dull in w200