Ah! Figured it out. For single the 2017 base of 2,300 is the standard deduction of 6,350 less personal exemption of 4,050. Not sure why they go at it that way, other than if TP has no income other than wage income that doesn't vary much during the year they will always have a refund. If they claim 1 on W4, the refund would work out to marginal rate x personal exemption of 4,050.

The 2018 base of 3,700 is 1,400 higher than 2017. This comes from the amount without HR1 would have been 50 higher at 2,350 (6500 - 4150). Then they added 1,350 which is the difference of new 12,000 less 2018 amounts before of 10,650 (6500 + 4150).

So, in the case of the simple return of standard deduction and only W-2 income, 2018 refunds will be slightly lower. For someone in 15% marginal refund of 607 (4,050 x .15) will go to 498 (4,150 x .12).

Works out the same for married if you presume 2 exemptions.