It isn't a quirk. It is very purposefully designed. The 33s has three entry systems:

1. The ALG mode, which is infix for operators and y^x, and postfix for functions (essentially 1970s algebraic) but with the very interesting and useful upper line showing the expression compiled!

The second system is "left to right" and is the equation editor.

The third is of course RPN operating on the stack.

Note that in ALG mode there is a stack of sorts: variables x1 through x4 which can be recalled with the Rdown button. Read the manual to understand their use.

Also note that Lastx can be used in Algebraic to do chain calculations between two running calculations. (Very early ROMs were somewhat different and I can't remember where I filed my write-up of that!).

I very much like the design of this machine. I wish that rather than retiring it, they simply perfected it. If it had the equation editing capability of the 35s, and all other minor bugs cleaned up (they already cleaned up the major ones) it would be terrific. Compared to the 35s, it has much better direct keyboard capability and it has a much better implementation of Rectangular to Polar and of BASE arithmetic.

The 35s has a few "special" ideas such as the vector business, but at the expense of the good stuff.

*Edited: 16 Mar 2012, 10:28 a.m. *