I have a theory about this based on a few examples I've worked with but please bare in mind its only a theory. Lowering springs for clios are typically sold as for non-sports (i.e. will fit all 1.2,.1.4,1.6,1.5 Dci) or for sport (172, etc). Now there is a significant difference in weight across the non-sport engine rage - especially taking a 1.2 vs. a 1.5 dci. This means there will be a small variance in the results if you fit the same springs to these cars. I reckon the 60 front 40 rear is a set of springs that on the smallest engines produces a flatter overall height.
A friend of mine fitted 50mm (all round) springs to his 1.2 8v (the lightest engine) and the front is noticeably higher than the rear. Whether it is as much as 20mm higher at the front, I can't confirm. We had a long discussion after we fitted them as the whether the 60/40 option might have been better on his car. Saying that, I don't think Ive seen any examples of clios on 60/40s.
When I fitted eibach sportlines (claim 45mm all round) to my non-sport I swear the front was a bit higher but I ruled it down to the fact I had a silly heavy subwoofer in the boot. Saying that, I highly recommend Eibach Sportlines for your spring option. They are a good-low and don't compromise the ride quality by much. They are bundles of fun in the corners too