r - as.numeric is rounding positive values / outputing NA for negative values

I am trying to do something with [R] which should be extremely simple: convert values in a data.frame to numbers, as I need to test for their values and r does not recognize them as number.

When I convert a decimal number to numeric, I get the correct value:

> a <- as.numeric(1.2)

> a
[1] 1.2

However, when I extract a positive value from the data.frame then use as.numeric, the number is rounded up:

> class(slices2drop)
[1] "data.frame"
> slices2drop[2,1]
[1] 1.2

Levels: 1 1.2
> a <- as.numeric(slices2drop[2,1])
> a
[1] 2

Just in case:

> a*100
[1] 200

So this is not a problem with display, the data itself is not properly handled.

Also, when the number is negative, I get NA:

> slices2drop[2,1] <- -1
> a <- as.numeric(slices2drop[2,1])
> a
[1] NA

Any idea as to what may be happening?