This month s income projections are higher than last month s were. 最後last month s 為何要補上were?不能省略嗎？

The DO is followed by an adv cl. introdiced by "as if."
eg:-This month's income projections are---------Subject + vt
higher than last month's-------DO, the apostrophe ( 's) in grammar showing possession; the possessive case to fininsh the sentence already; .
"as if" it were (projected)---------DO followed by an adv.cl. introduced by "as if" to further introducing an adv clause of manner, with a past tense "were" in the adv.clause.

上面TOMING88已解釋了，原句省了的重覆projections的意思，因為內容的說法是比較兩月的"projection(s)"而不是"單一件東西的比較，因此英語人會"特別/習慣"地把這"東西"的意念要在句子中反映出來。

當然中文人看(教)英語句一般只會寫A is higher than B便是正確的例句，但當A並不是一件物件而是一種狀況/狀態/動態的東西時，便要留心這類寫法：A is better than B was.便不單是比較一刻或在時空中一點的意思，當中可能包括A今日的表現比B在以往的表現還要好。(注意A跟B不是現在在比賽，而是可能在說他的工作業績或年度報告。)