WP 31S/34S: H.MS+/H.MS- anomaly?

[2far]

Hi all, I was just going to buy the hardware when I was checking out some calculation examples on the WP-31s and WP-34s emulator with unexpected results (subtract 1 degree from 18 minutes):

0.18 [ENTER]
1 [H.MS-]
-0.416 (not -0.42??)
1 [H.MS-]
-1.42 (ok again, except when you start with 0.416 -- then you'd expect -1.416)

With other values (0.17, etc) it delivers the expected results. It doesn't matter if you use -1 [H.MS+] or 1 [H.MS-]
-- you can also -0.416 [ENTER] 1 [H.MS+] with result 0.18

Is it some kind of strange rounding bug? I tried searching the forums first, but found nothing.

Cheers from Zurich,
Greg

PS: not sure if this should have gone somewhere else -- new to this