it isn't a bug: all cables suffer distance-related energy losses (wiki).
Copper cables, for example, lose 1 EU every 5 blocks, so this means that if your wire is 4 blocks long it does not lose any energy (energy loss is 0.8 rounded down to 0). Energy loss is not based on the total amount of energy moving through the cable: a solar panel can output max 1 EU/t during the day, if you build a five blocks long copper cable and you connect at the end of it 4 solar panels (on the three free sides and on top of the last piece of cable) you are not generating 3 EU/t (4 EU/t - 1EU/t) but 0 (1EU/t - 1EU/t + 1EU/t - 1EU/t + 1EU/t - 1EU/t + 1EU/t - 1EU/t). Energy loss is based on the "size" of each packets, not the sum of them (so you can theoretically send 300 EU/t through a copper cable if every packet is smaller than 32EU). What the batbox is doing in your system is storing energy, preventing any energy loss and sending packets of 32EU/t (so after 5 block through a copper cable you still have 31 EU/t)
I apologize for my English.