我有一个如下数据集：
X Designname Authorname Votes AvgScore SubmitDate year day week.of.the.year wk.list before.after Print Challenge
1 1685 final-boss jameses_x 591 3.47 2013-04-01 2013 Mon 14 1 before 1 Threadless
2 73684 favella yooka 348 2.72 2013-04-01 2013 Mon 14 1 before 0 Threadless
3 73687 watch-your-back-2 sayahelmi 481 3.15 2013-04-01 2013 Mon 14 1 before 0 Threadless
4 73688 smoking-kills-5 Professor Vince 379 2.19 2013-04-01 2013 Mon 14 1 before 0 Threadless
5 73689 meet-professor-vince Professor Vince 339 2.07 2013-04-01 2013 Mon 14 1 before 0 Threadless
6 73690 master-of-disguise-2 Mundara 331 2.81 2013-04-01 2013 Mon 14 1 before 0 Threadless
7 73691 lion-15 a7md 401 3.08 2013-04-01 2013 Mon 14 1 before 0 Threadless
8 73692 zen-attitude kharmazero 368 2.95 2013-04-01 2013 Mon 14 1 before 0 Threadless
9 73694 turn-the-other-cheek-3 Laurence Jenkins 303 2.26 2013-04-01 2013 Mon 14 1 before 0 Threadless
10 73695 fantasy-park zsonzsy 519 3.43 2013-04-01 2013 Mon 14 1 before 0 Threadless
11 73696 swiss-plumber diVulture 330 2.92 2013-04-01 2013 Mon 14 1 before 0 Threadless
12 73699 impala 108 372 2.95 2013-04-01 2013 Mon 14 1 before 0 Threadless
13 73700 catastrophe-divide-by-zero-2 Shadyjibes 323 2.74 2013-04-01 2013 Mon 14 1 before 0 Threadless
14 73701 catastrophe-divide-by-zero-2 BootsBoots 323 2.74 2013-04-01 2013 Mon 14 1 before 0 Threadless
15 73702 inking mainial 465 3.31 2013-04-01 2013 Mon 14 1 before 0 Threadless
16 73707 chloroville Sebcbien 304 2.67 2013-04-01 2013 Mon 14 1 before 0 Threadless
17 73708 warter-guns Gamma-Ray 413 3.21 2013-04-01 2013 Mon 14 1 before 0 Threadless
18 73712 labori BRLNVCTM 231 2.05 2013-04-01 2013 Mon 14 1 before 0 Threadless
19 73715 kunimitsus-mask FlourKirby 307 2.51 2013-04-01 2013 Mon 14 1 before 0 Threadless
20 73722 dmantis cuss pahing 322 2.94 2013-04-01 2013 Mon 14 1 before 0 Threadless
我想获得每个Authorname的最新SubmitDate（每个作者的最大日期）。
我尝试过以下代码：
agg = aggregate(SubmitDate ~ Authorname, data = sub.before.design, max)
看起来这行代码正常运行。
然而，agg中的作者数量与原始数据框中的作者数量不匹配。
在原始数据框中，我有超过11,000个独特的Authornames。在agg中，只有大约8000位作者。
我想知道我哪里出错，或者是否还有其他办法可以获得我想要的结果。