LH is much stricter with its carry-on rules as of earlier this year. Lots of discussion about this here (on LH forum):
http://www.flyertalk.com/forum/miles...-feedback.html
In March, when I was returning DUB-FRA-IAD I had to check my small rollerboard (which weighed at less than 20lbs). Check-in agent said it was the same rules or LH HON as well! So travel forewarned!