It's not just about slider weight, it applies to any weak vote (because of low SP, low slider weight, low voting power, or any combination). The reason is that Steem amounts come in exact multiples of 0.001 STEEM. If you're due a curation reward of 0.0099, you'll only get 0.009, which is a 10% loss. On the other hand, if you're due a curation reward of 1.0099, losing that last 0.0009 doesn't matter as much.