Do these sound levels seem strange?

Hi all. I am a newbie when it comes to configuring HT audio and it's my first time calibrating the speakers with a SPL meter. I have the following audio components.

Onkyo TX-NR807 receiver

Older Klipsch speakers
KSF 10.5 Towers
KSF-C5 Center Channel
KSF-S5 Surrounds

SVS PB13-Ultra subwoofer (on order as of today)

I calibrated my speakers using the AVR test tones and my RadioShack SPL meter mounted on a tripod pointed toward the ceiling. The meter was situated equal distances away from the mains and also equal distances between the surrounds. To even them out at reference levels, it ended up looking like this in my receiver setup.

Front Left: -8dB
Front Right: -10dB
Center: -5.5dB
Surround Left: +1.5dB
Surround Right: -4dB

Now I can see having some variations between speakers, but what surprises me is that 5.5dB difference between the left and right surround speakers. Does that seem strange? If so, what could cause that?

The readings were taken close to ear level in the main listening area. The surrounds are actually mounted on the side walls. There aren't any curtains, but the left surround is on the edge of wall that leads out to a hallway around the corner (if that makes a difference).